The Kitten, the Witches, and the Bad Wardrobe (The Kitten Board) is a forum dedicated to Willow and Tara. It is, to my knowledge, the largest, and certainly the most active, W/T-focused community on the Internet. There are more completed fics and works in progress here than I can count, as well as fan-created movies, icons, comics, wallpapers, art, etc. Plus, the authors of some of the best fan fiction around still hang out here and chat. I can not recommend this site highly enough.
Through the Looking Glass is an archive of Uber fiction featuring Willow and Tara. That is, stories in which the characters are retained, but that take place in different time periods and settings, having no overlap with Buffy canon. The quality of writing for many of the stories found on this site is superb, including Chris, the site owner's, own. Through the Looking Glass continues to be updated, and contains both completed fics and works in progress. Chris graciously allowed me to host his stories on my site, and he created many of the graphics you see here.
The Mystic Muse is the first Buffy fan fiction site I ever found. It contains many Willow/Tara stories, and allows you to sort by character pairing and author to quickly locate your favorites. The Mystic Muse continues to be updated, and contains both completed fics and works in progress.
Crypt of Fic is another Buffy fan fiction archive. Although it has not been updated since 2005, I have found a number of very good stories here that I did not see anywhere else. You may browse stories by character pairing and author. The selection of Willow/Tara stories is not as large as other sites, but the quality is very good.
That's my site! You're on it right now. I just wanted to show off this sexy banner that Chris made for me. If you want to use this to link to the Willow/Tara Fiction Archive, you are more than welcome to. And thank you! Please link to the home page.