Maharshi Vishvamitra (महर्षिः विश्वामित्रः)
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Vishvamitra was a great sage known for his austerity and mystic power. He was the son of Gadhi, a powerful king of the province of Kanyakubja (part of Uttar Pradesh), therefore also known as Gadhi-suta. Although he was a kshatriya by birth, he became a brahmana in the very same body by the power of his adhyatmik achievements. He was also one of those who tried to stop the Kuruksetra war.
There was a king named Kusha, a brainchild of Prajaapati, and Kusha's son was the powerful and verily righteous Kushanaabha. One who is highly renowned by the name Gaadhi was the son of Kushanaabha, and Gaadhi's son is this great-sage of great resplendence, Vishvamitra.
प्रजापतिसुतस्त्वासीत् कुशो नाम महीपतिः | कुशस्य पुत्रो बलवान् कुशनाभः सुधार्मिकः || १-५१-१८
कुशनाभसुतस्त्वासीद्गाधिरित्येव विश्रुतः | गाधेः पुत्रो महातेजा विश्वामित्रो महामुनिः || १-५१-१९
(Valmiki Ramayana, Balakanda, Sarga 51)
Conflict with Vasishta
Vishvamitra was defeated by the adhyatmik strength of Vasistha, and thus the king decided to become a brahmana, but Vasistha would not agree to accept him as a brahmana. In this way there was always disagreement between the two. Later, however, Vasistha accepted him because of Vishvamitra's quality of forgiveness.
When Vishvamitra was a kshatriya king, he visited the hermitage of Vasistha Muni, and he was given a royal reception. The king was surprised how can a mendicant be so opulent. He came to know that there is a cow named Nandini, who is protecting herself and others, and providing for all the necessities of the sage. Vishvamitra said give me this cow as I am the king, but Vasistha refused to deliver it. The sage said this cow represents all our piety, prosperity, tapasya and you cannot take her away. The king forcefully tried to take the cow away, and thus there was a quarrel between the sage and the King. Vasistha muni said to the cow I am not capable of protecting you, please protect yourself. Many soldiers became manifest out of her, and beated up all of Visvamitra’s soldiers. Vishvamitra became very annoyed and uttered - dhik balam ksatriyam balam. To hell with the power of a kshatriya. From there he started doing his tapasya. He underwent severe austerity on the bank of the river Kausika.
Vasistha was tormented so much by Visvamitra muni and at one point he thought of commiting suicide. The situation became extremely painful, when Vasistha muni lost his sons and grandsons, so he tied a rock and entered into river to die but the river washed him ashore. Thus his suicide attempt was unsuccessful. As per destiny, he could not die. He had a lot to contribute. Later in the dynasty of Vasistha, Vyasadeva took birth.
Once Harishcandra performed a yajna for which Visvamitra was the priest, but Visvamitra, being angry at Harishcandra, took away all his possessions, claiming them as a contribution of daksina. Vasistha, however, did not like this, and therefore a fight arose between Vasistha and Visvamitra. The fighting became so severe that each of them cursed the other. One of them said, "May you become a bird," and the other said, "May you become a duck." Thus both of them became birds and continued fighting for many years because of Harishcandra. 
Vishvamitra and Menaka
The great mystic Vishvamitra Muni, even in the stage of samadhi, became a victim of the senses and cohabited with the heavenly damsel Menaka. The mind, although ceasing to think of sensual activities at present, remembers past sensual activities from the subconscious status and thus disturbs one from cent percent engagement in self-realization.
His meditation was broken when he heard the tinkling of bangles on the hands of Menaka. He fathered a child who is universally celebrated as Sakuntala, the beauty queen of world history.
Trishanku is the celebrated name of Satyavrata, the son of Tribandhana. He kidnapped the daughter of a brahmana when she was being married. His angered father cursed him to become a candala. Thereafter, by the influence of Vishvamitra, he went to the higher planetary system, the heavenly planets, in his self-same body, but because of the prowess of the devatas he fell back downward. Nonetheless, by the power of Vishvamitra, he did not fall all the way down; even today it is mentioned he can still be seen hanging in the sky, head downward. 
Sons of Vishvamitra
Visvamitra had 101 sons, of whom the middle one was known as Madhucchanda. Fifty were older than Madhucchanda and fifty younger. In relation to him, all the other sons were celebrated as the Madhucchandas. Visvamitra accepted the son of Ajigarta known as Sunahsepha, who was born in the Bhrgu dynasty and was also known as Devarata, as one of his own sons. When requested by their father to accept Sunahsepha as the eldest son, the elder fifty of the Madhucchandas, the sons of Visvamitra, did not agree. Therefore Visvamitra, being angry, cursed them. "May all of you bad sons become mlecchas," he said, "being opposed to the principles of Vedic culture. Thus the younger Madhucchandas accepted Sunahsepha as their eldest brother and told him, "We shall follow your orders." Visvamitra then said to his obedient sons, "Because you have accepted Sunahsepha as your eldest brother, I am very satisfied. By accepting my order, you have made me a father of worthy sons, and therefore I bless all of you to become the fathers of sons also." Visvamitra cursed some of his sons and blessed the others, and he also adopted a son. Thus there were varieties in the Kausika dynasty, but among all the sons, Devarata was considered the eldest.
Kausalya Supraja Rama
This verse is originated from balakaanda of Sri Ramayan, where Vishwamitra wakes up Sri Ram in the morning citing these words. Some acaryas regard this verse as good as the Gayatri mantra.
These are the opening words of the Sri Venkateswara Supbratham to awaken Lord Venkateswara (Balaji). This is the first sloka rhymed while the temple doors are opened everyday morning for the first time at Tirumala. The stanza starts as:
कौसल्या सुप्रजा राम पूर्वा संध्या प्रवर्तते | उत्तिष्ठ नर शार्दूल कर्तव्यम् दैवमाह्निकम् || 1-23-2 
Fortunate is Kausalya to beget you as her son Rama; get up oh, tigerly-man, eastern aurora is emerging, daytime tasks towards gods are to be performed.
In the scriptures there is mention of äpad-dharma, or occupational duty at times of extraordinary happenings. It is said that the great sage Vishvamitra went to a candala’s house to eat the leg of a dog in a extraordinary dangerous situation (during a drought which lasted 12 years). Candala told to Visvamitra muni that even if you lose your life, do not give up your culture. Visvamitra muni convinced him by saying that I can digest the flesh and the karma (sinful reaction) also. However, at that time Indra showered the rain from above.
The principle is that in cases of emergency, one may be allowed to live on the flesh of animals of all description, but there was never a time in vedic age when there was encouragement by the state for maintenance of regular slaughterhouses, where numerous animals are murdered wholesale and mercilessly.
Exceptions are not general rule
From the history of Parasurama it is seen that although Parasurama belonged to the brahminical group, he circumstantially had to work as a kshatriya. Later, after finishing his work as a kshatriya, he again became a brahmana and returned to Mahendra-parvata. Similarly, it is seen although Vishvamitra was born in a kshatriya family, by austerities and penances he achieved the position of a brahmana. This exceptions in sastras can never be regarded as a general rule for others to implement. Prescribed duties are duties enjoined according to one's psychophysical condition, and one can live more naturally if one sticks to those.
Such practice is not encouraged by Lord Krishna also in Bhagavad Gita wherein He says - It is far better to discharge one's prescribed duties, even though faultily, than another's duties perfectly. Destruction in the course of performing one's own duty is better than engaging in another's duties, for to follow another's path is dangerous. 
- Srimad Bhagavatam, Canto 9, chapter 7 verse 7 Harishcandra
- Srimad Bhagavatam canto 9 [7.5-6] The descendants of Mandhata
- Srimad Bhagavatam Canto 9 Chapter 16 Madhucchandas The sons of vishvamitra
- A.C. Bhaktivedanta Swami Prabhupada (Bhagavata Purana) The Bhaktivedanta Book Trust International, Commentary on Srimad Bhagavatam 1.17.16
- Bhagavad Gita 3.35 Bhaktivedanta Vedabase