Not sure what you need. If you're talking about portal gadgets, there is the one Data-Gadget as well as the IAC Gadget for embedding in other applications.

The more general use of "Gadgets" apply to different functionalities where a Gadget is a group of rules - probably always containing at least one Section rule and probably others. So you could filter on Section rules :

Probably the best list you could want is from App Explorer ... seeing what extends from PegaGadget.