Dittois the best clipboard freeware out there and is the only one that fully met my specifications. It's simple yet powerful, extremely fast, light on resources, it stores unlimited clips in a sqlite database and it has hundreds of settings to play with. To name a few of its features: it has a search box that enables you to find a 3-day-old clipboard within seconds; 3 different virtual clipboards that you can access through dedicated shortcuts; and it has unique network capabilities, making clipboard sharing a reality. Ditto is actively developed, it is more robust, efficient and flexible than most of its competitors. It will fit the needs of both beginners and advanced users, no matter how the use their computer. Ditto has been my main clipboard replacement utility for almost two years, and it has never crashed yet.
CLCLis a simple and consistent clipboard manager that works particularly well with older windows versions, and is very easy to use with either keyboard or mouse. Its development stopped in 2005, and you have to install it manually, but CLCL's strong supporters are proof that it is worth the trouble.
ClipX is a clean and simple clipboard manager. Plugins are available to bring some interesting features like a color picker to copy the code of a color, stickies (permanent clips), "search in google" feature... It is not developed anymore (the latest beta was released in 2008), but it remains a very good clipboard manager.
Clipboard Help+Spell is a promising clipboard manager, full of features. You can add notes quickly, timestamps, there are powerful text edit features and a spell checker... It is actively developed but it has one major flaw: it does not handle anything else than text.
ArsClip is a good option for those looking for a portable clipboard manager. In the zip file, you'll find the portable version, the setup.exe and the source code.