You must have
TinkerTool. It's a simple and extremely useful interface to adjust preferences that Apple has built in but doesn't let you fiddle with--for example, putting double scroll arrows at both ends of a scroll bar, which I've used for years despite it never being an "available" feature. And, it's free.
I like the small freeware program
ASM as well; it provides some additional application switching functionality, particularly forcing all windows from an app to the front when you activate it. Not for everybody, but I like it.
[edit]
Egad! I almost forgot the single most useful piece of freeware I've ever seen:
Cocoa Gestures. If you've never heard of gestures, you can basically set a specific mouse button (or modifier key) to let you draw a "gesture" with your mouse that will trigger some action.
For example, click the thumb button and drag left a bit to go back in a browser, or click and drag down to close the current window/tab. It's done more to make using my computer easier (particularly web surfing) than any single application I've used in the past 10 years.
Not only is this little program free, but it's configurable on a per-application basis, lets you apply gestures to any button or modifier key (including buttons that also do something else, like right click--even that works fine), and gives you full control over gesture programming and can trigger any menu action.
This allowed me to give some favorite pages (dictionary, for example) a gesture (draw a "D", and I'm there), and do the same for changing language encoding (which is usually buried two menu levels deep and has no key shortcut--no just draw a "J" and I'm in Japanese).
Why more people haven't heard of this, I don't know.