Source Code Browsing Tools?

Marco Sanvido asks: "I often look at source code (especially C, but this question is valid for other languages as well) and I have a really hard time in understanding how it works. Documentation is often missing or quite outdated, and the only way to see how the program works is to try to understand the source code. Which tools do you prefer to use for browsing and studying source code? So far I have used LXR for Linux, Eclipse for java, and CScope, but I'm not sure that these tools are the best solution." It's tempting to flood this question with answers for your IDE, but the key thing here is _browsing_, not _development_. What decent, lightweight programs would work well as source code viewers?