I don't know what you need specifically, but the default install of synaptic (once you hit the 'Reload' button) offers a lot of stuff! If you can't find it there it may not be worth using?
My experience is that if you use too many package install apps or .repo's then you'll wind up with a corrupt system. If youwant to add an app from a specifis repo then I recommend installing 'Yum Extender' for FC3. Don't know what disto you're using? It's a GUI front end for yum/apt.
With this you can add repo's and turn them on and off easily. So if you find a weird app you simply can't do without, and add their repo, you can turn it on for the install and off afterwards thus not affecting your install.
Read about it here.