I find performance work to be incredibly fun and rewarding. Here's a great article on ten things Apple did to make Mac OS X faster. I love Apple's performance monitoring tools, especially Shark. Shark lets you see where time is being spent across all applications. It was invaluable for investigating performance issues in Mac Office, especially cross-process issues (like our database daemon) that can't be easily profiled by tools that look at a simple application. I still haven't had time to investigate performance tools on Windows much. There are tons of great tools at http://sysinternals.com, although nothing quite like Shark. Can anyone recommend some great Windows performance monitoring tools (especially ones that don't require rebuilding apps)?