I lied. We'll save the defrag tools for another time.
Right now, let's focus on a tool almost everyone on here can use. Steam Cleaner.
http://www.fpsbanana.com/tools/3595
What this handy little tool does is clean out all the crap saved each time you play on Steam.
BE CAREFUL WITH THIS TOOL. If you don't know what something does, don't touch it.
It supports Counter Strike Source, Team Fortress 2, Garry's Mod, Day of defeat: Source, and more.
Why should you use it?
I'll tell you why. Every time you play on any server, temporary files are stored on your computer. Steam doesn't really need them, it just likes to keep them. Steam is an obsessive hoarder that refuses to attend an intervention. Left on its own, it will keep saving enough junk until it buries itself in its own clutter.
Enter the cleaner.
What I normally do is choose the game I want cleaned, tell it to clean the Game Overlay and normal crash logs (who needs those, really?), the temporary spray files (I don't need backup files of every spray I've ever seen), and sometimes I'll remove temporary downloads.
All the files go straight to the recycle bin.
If you are an admin, I advise NOT cleaning demos and screenshots.
Don't even think of touching the "Remove sounds, models, textures, maps, derp" settings. They bring pain and destruction.
Use this tool, and Steam will be a bit quicker, and your hard drive will thank you.