I’m a big fan of GrepCode. It’s a code search engine optimized for Java. The engine crawls various Java open source repository and parses the source code it finds, creating a deeply hyperlinked representation. It’s almost as good as an IDE for navigating third-party code, but you don’t need to find that source code yourself.
In the past I had high hopes for Google Code Search and Krugle.org. Both had the goal of being a broad index of open source code in all programming languages. The former was discontinued and the latter today has such poor coverage that’s its nearly useless (sorry Ken). GrepCode is limited by being Java-only, but it’s soooo good.