The locations listed may change at any time. Your best bet is to enter the name of the series plus your preferred download method (usually torrent) into your search engine of choice. For instance, to download One Piece you may wish to try “One Piece Torrent”. This listning currently gives me the majority of locations where you can download One Piece.
General Anime Download Locations
Unlicensed anime is located here in mass quanity. For instance, right now you could easily download all of Busou Renkin.
An age old favorite, AnimeSuki provides an excellent listing of unlicensed anime from various groups in a searchable format.
Another one that has been around a long time, the majority of anime you find on various sites that is aged any (for instance, Kenshin or Hikaru no Go) can be found here. Only carries FULL seasons of shows, so generally a show will be finished airing in a giant block before arriving here.
Similar to AnimeSuki, although it lists quite a bit more. This is a good place to keep track of newer anime.
Fansub Group Listings
This section only lists a few of the big ones, as fansubing groups generally go down often. Fansubing groups are the BEST place to look for XDCC bots with unliscensed anime and you can generally find their location by googling both their name and the series they handle. For instance, ADC-Elites and One Piece will generally lead you to their site.
Made popular by their subs of Kalido Star and FMA, Shinsen is a decent fan subbing group that currently handles Gintama (although, very slowly).
GG came onto the fansubing scene with Poni Poni Dash and have since fansubbed various series. Their biggest claim to fame so far is Code Geass.
One Piece is mostly handled by Kaizoku Fansubs (with DVD releases) until they run out, then you can take up on ADC Elites then Vegapunk (who seems to be doing faster then ADC). This of course, is subject to change.