Now that I've explained what you may (or may not) need on your Mac; Here are some extra Preference Panes that I have running and now take for granted...
1. Shades - Yes, Shades was on the list of must have applications. But that's what's cool about it; It can be in your Dock, randomly on your screen, in the System Preferences panel or on the menu bar.
2. Detour (Application Enhancer) - For lack of a better way to put it; Application enhancer is required for Detour. So I don't actually know what it does. But Detour, on the other hand, is a preference pane that maintains my sanity. For instance, if I had a flash game running and I couldn't turn off the sound, and at the same time wanted to listen to my own music. I could tell Detour to turn off (or down) the sound to FireFox and leave iTunes turned up.
3. Divx and Flip4Mac - Two programs I despise the very guts of and yet can't live without. I appreciate the quality of Divx files. But every time I play one in FireFox, FireFox becomes the only application. I can't see any other applications in front of FireFox. Not even the Dock or the Force Quit menu. Flip4Mac is a requirement because it allows me to view WMV without Windows Media Player. But every time I use Quicktime Pro to export or save a WMV as another format it puts big huge letters over the video saying "Created using Flip4Mac" almost as though it was important.
4. iTheater - Just a little application I don't use very often but is a wonderful replacement for Front Row.
5. Perian - Quite possibly the one application I couldn't live without. Have you ever tried to open a video and it said you didn't have the proper codex? Then it directs you to a site with a list of codex and you don't know which one you need? Perian is all of those codex(plural) and more. With Perian you can view and (with Quicktime Pro) convert anything (except WMVs, hence Flip4Mac. Blah). You install it and never notice it again.
6. AppTrap - Yes it was also on my other list but it is also a preference pane.