strace is one of my favourite tools. I don't need it that often, but it's comforting to know that it's there. Documentation can lie or just not answer my question, the source code might be unavailable or unreadable, but if I have a question that strace can answer, I can rely on it to answer.

It's one of the things I miss on OS X. dtruss isn't as powerful, and dtrace seems like it might be more powerful but I have no idea how to use it.