Solidity is the most widely adopted language, but there are other choices including Serpent (Python-like)and LLL (lisp-like).

A few developers I know use Serpent because it is simple and stable (meaning it has not been changed or updated for a while, unlike Solidity which they say is continually being tweaked in a manner that can be aggravating).

Interestingly, Vitalik's preferred language (say for prototyping Casper algorithms) is Python. He created Serpent and is now working on Viper, which sounds like a next-generation Serpent. Viper is being implemented in Python3.

You can see the work in progress at: https://github.com/ethereum/viper