Thank you for the information above, but yes. What applications / utilities / system hacks / whatever are running? What OS version?
Try to run Activity Monitor (or top from the command line) and sort by % CPU. See what your highest items are... When you are not doing too much actively, you should usually see just a few items with <25% or so of CPU including the focused app and maybe a couple of system items. If anything shows up as taking >75% of CPU, then there might just be a task that has gone wild for some reason. You can often kill the task and it will just restart. Depends on what it is. It could be associated with a bad preferences file, which won't be solved by anything on your to-do list. But if you know what program it is, you can spotlight for the preference file and delete it.
Also, if that does not turn out to be the case, and this is a Tiger thing... try for a moment, shutting down Safari and seeing if other programs become snappier when Safari isn't running. I have had this weird issue recently where Safari occasionally causes the whole system to gum up. I have not been able to figure it out. Sometimes, the Reset Safari menu item solves it temporarily. Other times, I end up getting frustrated and using Firefox for a while....