TinkerTool is a small, free app that lets you tinker with your user preferences, allowing access to options that you can't find through System Preferences. These "hidden" preferences--extras like adding scroll arrows to both ends of scroll bars--exist for a variety of reasons, but are usually invisible to users without the help of an app like TinkerTool.
TinkerTool's preferences are organized under 10 main headings: Finder (for options like showing hidden files or restricting the Finder), Dock, General (a lot of useful options, including control over scroll arrows and screenshot file formats), Applications, Snow Leopard (with options for Spotlight, Mail, and Spaces), Fonts, Font Smoothing, Login Items, Safari, and QuickTime X. Compared with some other "tinkering" apps, TinkerTool is a bit more limited in its options--but that limitation can also be reassuring, since TinkerTool doesn't add files, run background processes, or require any special permissions. If you get tired of TinkerTool, you can choose to reset your computer to its "pre-TinkerTool state" (or not) and simply trash the app. Recent builds have kept TinkerTool up to date with the most recent changes in the Mac OS and related apps, including the capability to disable the Ping and iTunes Store pull-down menus in iTunes 10, and disable all RSS features in Safari.
You can find similar apps with more potent features, but TinkerTool is a solid (and free) option that's worthwhile for anyone with limited preference-tweaking needs.