The natural successor to TrueCrypt (unless the guys at TCNext can come up with something better) is VeraCrypt. The (French) developer took TrueCrypt's code and fixed the weaknesses found during the security audit performed on TC. The main enhancements and corrections done so far are listed on this post.

VeraCrypt's dev is also very active on the TCNext forums with the username IDRIX, and unlike the devs of TC, who never revealed their identities, he isn't afraid to reveal his real name, picture and personal info. On this topic, he answers lots of questions related to VeraCrypt.

The second free and open source alternative I'd recommend is DiskCryptor. It's main drawback is that, unlike TrueCrypt and VeraCrypt, it's only available for Windows. Also, it's developed by anonymous Russians, so there's that.