A number of these were recced over at veni_vidi_vids, or posted at vidding, which is how I found them in the first place. This list will be updated the more I watch, with the videos that I think are especially memorable. Fandoms are ordered alphabetically, within the subsections vids are in the order I encountered them in.

Avatar: The Last Airbender

thedeadparrot's Paper Planes (M.I.A). Toph kicks some serious butt. So does this video, I love the sync of audio and video, especially in the chorus, and it's tremendous fun to watch, and a wonderful kick-off (heh) for this recs post.

bananainpyjamas's Heavenly (The Dandy Warhols). Zuko is a complex character, and this more than does him justice.

Yanny333b's Stream of Life (Palbasha Siddique), lyrics in comments here. Ensemble, spoilers for the entirety of Avatar, but the editing is completely amazing, and the vidder deserves major bonus points for using a powerful Bengali song based on the Gitanjali poems for an Asian-inspired fantasy setting.

Battlestar Galactica

chaila43's Cathedrals (Jump, Little Children). Caprica, an achingly beautiful video.

Doctor Who

kaydeefalls's Stray Italian Greyhound (Vienna Teng). The Doctor, Amy Pond.

diarmi's You'll Be Bright (Cloud Cult). The wonderful female characters of Doctor Who.

such_heights's Lullaby for a Stormy Night (Vienna Teng). W(h)ee!Amelia. :3

greensilver Change of Time (Josh Ritter). A Rory Williams character study.

Firefly

y_fish's Defying Gravity (Wicked). The women of Firefly. Watch it, seriously.

Lord of the Rings

rdphantom's A Thousand Years (Sting). Hypnotic and amazing Fellowship video.

wolfling's Winter (Tori Amos). Featuring Éowyn and Theoden, beautifully sad, and sheds an incredible light on their relationship.

tehlime's Into the Water (Laura Marling). Frodo's Quest and the burden he carries.

Return of the King, Uh Huh (Tom Smith). Just what it sounds like. Whacky fun using the animated LotR versions.

Stargate: SG1

kuwdora's I Wish I Was a Punkrocker (Sandi Thom). My second girlcrush in all her glory: Sam. SamSamSamSam. Awesomeness condensed into one amazing vid.

Pokerface (Lady Gaga). Yes, it's Lady Gaga. It's also perfect source, perfectly vibrant editing, and it's Daniel/Vala. Hence, divine. (And no, that is not meant to be a pun on her name. Or is it?)

Supernatural

secretlytodream's I Know I'm a Wolf (The Young Heretics). Sam's Dark Side (with a possible AU ending).

secretlytodream's The Roaring of Lions, the Howling of Wolves (Thrice). Another dark Supernatural AU. Dean is telling Sam to kill.

The X-Files

laurashapiro's Rook (XTC). A Mulder character study and his search for answers.

laurashapiro's and killahbeez Not Only Human (Heather Nova). The Scully counterpart to the above.

Misc/Multifandom

All the Right Moves (One Republic). Period Drama Dance. Upbeat eyecandy, brilliant. *squee*

eruthros's and thingswithwings's One Night Fandoms (Angel of the Morning). A multifandom tribute to Yuletide, but wonderful even in August.

arefadedaway's One Girl Revolution (Superchick). Multifandom. Women are kickass, and this vid makes it abundantly clear.