Windows: ClipCube is a little clipboard history tool that lives in your system tray, silently keeping track of everything you copy for as long as you'd like.
ClipCube is very straightforward: you launch it and it starts recording your clipboard history indefinitely. If you'd like it to purge the history after a given period of time, you can configure that (as well as other options) by right-clicking its icon in the system tray. A left click on the system tray icon will bring up ClipCube's clipboard history for viewing, letting you copy any part of your history back to the active clipboard. If a URL was in your history, you can also click it to open it up in your default web browser. ClipCube stores its data in a folder beside the location of the app, so if you want to move your history to another machine you can do so easily.
ClipCube is currently under development (although it worked problem-free for me) and runs on Windows XP, Windows Vista and Windows 7. It's available to download right now for free.