Various more or less helpful utility classes:
Assertion - Various "assertion like" NSException subtypes: IllegalArgumentAssertion, IllegalStateAssertion etc).
BiDictionary - Maps keys to values and values back to their keys.
MultiDictionary - A one-to-many dictionary.
NSData+Encoding - Encoding related (Base64, MD5) extension for NSData.
Preconditions - Useful assertions for checking method arguments.
Stack - Push/Pop/Peak.
Tuple - Two things in one.
UIImageView+Reflection - Image reflection effect similar to the album art effect used by Music.app (from Apple sample code).
VarintCoder - Serializes ints to one or more base 128 bytes.
XMLWriter - Easily build simple xml documents.