TuxGuitar for opening and editing power tabs, making PDFs of tabs, midi etc.
I’ve found a couple of neat tab places specific to bands themselves, but they’re not open source as much as maintained by one or a few people - nonetheless not UG :)
Sonic Youth have an officially hosted tab place on their site.
The Fall have tabs hosted on what I think is a fan site :)
I’m guessing the reason there’s not one already is the sheer number of copyright issues that would appear the second it’s online - Ultimate Guitar made some kind of deal if I remember correctly, which is probably why they’re so aggressive about making me download the silly app and buying premium. :/