One thing I love about ruby is that all of the conference's videos are posted online , so that people that could not attend can watch them. I'm sure most of you know about Confreaks , which offers them for download.
Are there sites like that for other programming languages/conferences (or is google video the best shot)?
MSDN webcasts are all available on demand. Not exactly conferences, but essentially hour long presentations like you'd attend at a conference.
Apple posts a few videos on their dev site, but all of the latest ones (WWDC07) require you to be a paying member.
The Chaos Communication Congress has some of the best hacker videos out there. The last three years of videos are available online:
http://chaosradio.ccc.de/22c3_m4v.html
http://chaosradio.ccc.de/23c3_m4v.html
http://chaosradio.ccc.de/24c3_m4v.html
The Google Tech Talks are often amazing. The TED conference, while not strictly about development, are still quite impressive and a must see, if you ask me.
Im Not sure this answers your question but i ll post it anyway
Microsoft PDC : http://www.microsoftpdc.com/
Microsoft Mix : http://videos.visitmix.com/
The Yahoo! Developer Network has some great videos up. Some good javascript ones, especially Douglas Crockford's (there are more than these 2).
There are a number of video (and audio, of course) podcasts that cover developing very well, including walk-throughs of various technologies (ROR helped kicked that off, of course). For podcasts, Itunes is one excellent source (also where I download TED.com videos via as well).
I've just found an interesting list on mefeedia.com/video/tech but another I've seen several presentations from (usually audio, with slideshows) is infoq.com
For audio-only podcasts, there is, of course, StackOverflow, and also the longest running such site - IT Conversations.
The pycon people have posted a TON of video from pycon 2009. http://pycon.blogspot.com/2009/04/all-pycon-2009-videos-uploading.html
I wonder why noone hasn't mentioned DEF CON yet. According to wikipedia the world's largest annual hacker conference. Starting from DEF CON 16, the videos are in HD and split screen, i.e. you see the speaker as well as the slides, which makes the video much more enjoyable imho.