Here are lists of extensions (from the default sources) that can add features to your installed apps. Cydia automatically builds these lists by looking for packages that seem to use Substrate to modify these apps. Note that it may list results that a human wouldn't list, and it may list results that aren't compatible with your iOS version. We found these lists fun to look at, and maybe you'll like them too. (The lists are presented in a random order, to give a fair chance to every extension. ;P)