Two things:

On Tiger, you'll intermittently notice your machine huffing away. Last few times i've checked top to see what this is, i've noticed a mysterious MDS process taking up 60% CPU. Not good. MDS appears to be Spotlight's meta data indexing process. It really hogs the cycles when it runs, which is usually just a brief period, thankfully. Still, if you're developing applications on your machine it can be a bit much. More on this excellent Tiger bug-tracking blog or this MDS thread. Don't be afraid to just kill [pid] if it insists on hanging around - it doesn't affect anything other than Spotlight by the looks of things.

Also,

Dashboard is the biggest pain in the perineum. It's a glossy, promotional Tiger feature that leaks. The widgets display a level of interface usability befitting of a Java GUI. And it's always on. Get rid of it.