As glad as I am that we have NSAttributedString on iOS now, creating them can still be a bit longwinded. Chris Devereux has created this handy class for super simple markup based creation of attributed strings with easily specified styles for each markup tag.
I don't know about you but I always write a class to encapsulate my NSUserDefaults access but it's always written custom for each project and while it isn't hard to do it is dull, repetetive code. GVUserDefaults is a simple class that you can subclass and add @dynamic properties to to automatically read/write defaults. No primitive type support as yet but you can still add your own properties so it's not a huge problem.
UILabel subclass from Sebastien Thiebaud which can highlight and provide interactivity around hash tags, @usernames and URL links. The clue is in the name as to what this was originally intended for but these conventions are used across a large number of services now. Useful.
It's been a while since I linked to a Friday Q&A but I found this one particularly interesting. A great article on some common (and not so common) pitfalls to avoid or at least be aware of when writing Objective-C.