Sparkle is great. We use it in the Conveyor packaging tool to supply the Mac update implementation for apps, and automatically integrate it into apps without needing them to do any code changes (using a mix of code injection and other techniques). It's definitely one of the more robust and well featured update engines out there.

Actually we use a fork of Sparkle that adds support for forced updates on start, which is useful for apps that need to stay in sync with a rapidly changing server protocol. It gives you a more web-like approach.