To get the best of you iDevice you must install and tweak your device. If you have a wide range of apps to choose this can be simple. But for this you will need a platform to access all those catalogs so download cydia for free. Most sources are based on themes, others on apps and tweaks. Adding repositories is simple you just need to load cydia, touch the manage tab and then hit Sources section. Right now you should see a list. This list is composed from repositories that you have installed on your iDevice. To manage them touch edit, if you want to delete a repo touch the red dot with the white line to delete the respective source. To add one hit the Add button on the upper left corner and input the proper url of the repo and tap Add Source. The procedure of installing the repository will begin, when will finish hit Return to Cydia. To see the packages in that repository tap the respective source.
The list of all repositories is quite huge let me share a few of them that I consider good:
BigBoss & Planet-iPhone: http://apt.bigboss.us.com/repofiles/cydia/
Enjoy testing and if you have something to share please do not hesitate to do so.