Nila

When Lord Sri Venkateshawara (MahaVishnu/Balaji) was inside the snake-antHill on Tirumala , one sacred cow is to come and give milk to the Lord daily (Bhramha took the form as sacred cow), When a cow-herd man at once saw the sacred-cow pouring milk in the snake-ant Hill, he became angry and without understanding the truth, the Cow-herd man took the axe and has hit the cow on the cow’s head with the axe. The Supreme Lord Sri Venkatesha(Vishnu) who was present inside the snake-antHill received the hurt of the axe on his head from the cow-herd man instead of Cow getting hurt. When Lord received the hurt of the Axe on his head, portion of his hair on his head disappeared due to hurt by axe. When mother Neela Devi saw Lord being hurt on his head, she immediately removed the hair from her head and placed on the Lord’s head in the portion where lord was hurt. Immediately Lord became well and Lord’s hair reappeared as beautiful as before he was hurt. Lord Venkatesha(Vishnu) knowing that hair is one of the beautiful features of women, Lord Sri venkatesha promised that his devotees will shave Hair of their head (tonsure) and dedicate their hair to her. Neela Devi accepts the hair sacrificed by Devotees of Lord Venkatesha(Vishnu) and Lord blesses his devotees who sacrifice their hair for the sake of the Lord. This is the reason why Devotees tonsure or shave their head in Tirumala.   

There are also Spiritual reasons: Tonsure or Saving hair as offering for Lord represents a real sacrifice of material beauty and giving up false-ego in the way of shaving their hair as sacrifice to please Supreme Lord.

http://en.wikipedia.org/wiki/Nila_Devi

Discussion
http://www.ramanuja.org/sv/bhakti/archives/oct97/0077.html

Blog
http://samharshbangalore.blogspot.in/2013/03/a-hair-raising-story-of-tirupathi.html

Tube
https://www.youtube.com/results?search_query=nila+devi

Mountain
https://www.youtube.com/results?search_query=Neeladri+tirupathi