Four noteworthy ones that aren't too big & bloated:
1.
Http://bitmessage.org is awesome because of its' unique blockchain-driven design... It needs polishing though. If you use Mozilla Thunderbird for email, there's a plugin that helps with most of that.
2.
Http://Crypto.Cat is probably more what you're looking for, it's a lot like skype. Super easy to use, built into chrome, and tested to be very anonymous... But doesn't scale well so one day it could be overloaded and die. Don't have a clue what day that will be though.
3.
Http://kryptokit.com is a first-rate Bitcoin Wallet first and foremost, but it has a highly secure messaging functionality inside it too. -Not suitable for noobs; you have to share private keys with whomever you're talking too first which is never going to be accepted by the mainstream.
4.
Http://Jitsi.org is smooth, highly secure, and works with whatever IM client you already have like facebook or gtalk. It even secures videochats! -The problem is that it's still a bit technical for many users, but that's something they'll probably work out eventually. ...It's opensource, and has a huge international Dev team, but sadly it's a centralized service, not P2P, so one day I expect it to get taken down.
There are tons of bigger solutions too that try to offer more than chats and messaging, whole communities are starting to go onto the deep web behind awesome encryption.