Is there one that is the most popular? What kind of things does it add to the device? Does it improve the web browsing/mail/menu experiance?
My main problem with the droid is the interface seems to lag unlike the ipod touchs interface which feel as smooth as air.
There are a few popular ones, DroidMod, Petes, but it's all personal preference.
Off the top of my head, I can think of multitouch browsing, 2.1-based OSes, overclocking....
The Android UI is indeed different (less smooth) than the Touch or iPhone. This is for any number of reasons, but can mostly be attributed to multitasking. I really dont think there's anyone here who will tell you they find it unusable, in fact quite the opposite.
I'm willing to trade a little "smoothness" and "eye candy" for a phone that will let me do more than one thing at a time.