Earlier this week, there was a question posted of third party libraries that people use for iOS. It was closed, but had lots of responses. I think there should be a wiki post that contains all of the ones recommended here to point new users too.
First off, we've found that list-style questions don't work well in the format of this site. They generally devolve into piles of dozens, if not hundreds, of answers that aren't curated well. People won't read the whole list of pages of answers, so they just keep posting duplicates and the whole thing ends up being a mess. That's why that question was closed.
Second, this question has been posed many times in the past. Two of the ones I can find right now are
Here's the content of the question and answers. Feel free to edit them into the appropriate tag wiki or anywhere else that is appropriate.
MBProgressHUD is quite useful when you need a loading indicator
UnitTWebSocket - Websocket client