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. :/
Back in the early 2000s, we had MX Tabs which was not quite open source but amazing. It collapsed because of copyright issues. But while it lasted, it had basically anything you can think of. Usually 4 or 5 versions and they were all wrong in different ways.
Oh wow, that took me back! I used to learn bass songs on MX Tabs, but it was so long ago I forgot; I still have Nirvana tabs from MX that younger me printed out! :)