The Resistance (originally on scifi.com)
Battlestar Wiki; IMDb; Wikia; Wikipedia; YouTube

Based on the series Battlestar Galactica. This was a series of ten webisodes that aired online in 2006, between the second and third seasons of the show. It's about the Cylon occupation of the human colony on New Caprica.

Razor Flashbacks (originally on scifi.com)
Battlestar Wiki; IMDb; Wikia; Wikipedia

A series of seven webisodes that aired online in 2007, leading up to the TV movie "Razor" (which aired on Sci-Fi Channel between seasons 3 and 4 of Battlestar Galactica). I probably watched them, but now I don't really remember anything specific about them, nor have I much interest in them.

The Face of the Enemy (originally on scifi.com)
Battlestar Wiki; IMDb; Wikia; Wikipedia; YouTube

A series of ten webisodes that aired online in 2008-09. They aired during the hiatus between the first and second halves of the fourth (and final) season of the show. I must have watched them, but now I don't remember anything specific about the story. I probably liked them, though.

Blood & Chrome

This is technically a separate prequel webseries, which later aired as a TV movie. I haven't seen it, but if I ever get around to watching it, I'll write a review on its own page, since it's not a direct part of the Battlestar Galactica TV series. (I should include a link here, though.)