We have a thread for individual videos. But this is for channels. Discover a great channel put it here. I'll start off with some I've recently found. YouTube has recently made it easier to navigate through channels, rather than just video-hopping, sticking to one channel revolutionises YouTube in my opinion and is almost necessary if you want to avoid brain-numbing guff. Please recommend good ones that you currently subscribe to or have just found out about.

"Topic - Channel - Link" format please, then you can say why you think it's awesome, but that's optional. These are some sciencey ones I like to start off with.

Physics - Sixty Symbols - http://www.youtube.com/user/sixtysymbols Lectures by lecturers, who tone down the maths for the viewers, but not as much as TV 'science' shows. Watch if you like watching Horizon.

Interesting Facts - Vsauce - http://www.youtube.com/user/vsauce - watch if you like things like Mythbusters, but more like just reading a popular science book

Chemistry - Periodic Videos - http://www.youtube.com/user/periodicvideos - The chemistry version of Sixty Symbols

Maths stuff: http://www.youtube.com/user/Vihart
Car stuff: http://www.youtube.com/user/motortrend
More car stuff: http://www.youtube.com/user/drive
TED talk stuff: http://www.youtube.com/user/tedtalksdirector

this quirky guy is all you ever need:

http://www.youtube.com/user/tommync2010

No, that's an example of the brain numbing guff I was talking about

you dont like watching 22 year old americans who cant read or write talking a load of uneducated rubbish? his gangnam video is epic though

Ooh I forgot one:
General strip mine of information about most things: http://www.youtube.com/khanacademy

I'll branch out away from sciencey stuff soon, but behind Sixty Symbols and Periodic Videos is Test Tube: http://www.youtube.com/user/nottinghamscience

Adds back in some of the maths, for a more sober discussion on the topics. Looks also to be topical discsussion on the science issues of things in the news (often they correct misconceptions by journalists). I'm really enjoying this section of YouTube at the moment.

I'm subscribed to 54 channels. Very few would be of interest to most people on here I think, mostly being music related, cookery or just friends. There appear to be 3 mildly amusing ones after I filter those out:

Gamer Geek Comedy - The Guild - https://www.youtube.com/user/watchtheguild - mostly because I find the red head hot.

Observational Comedy - Community Channel - https://www.youtube.com/user/communitychannel

Animated Comedy - Amazing Super Powers - https://www.youtube.com/user/aspshorts

I thought I was subscribed to a Minecraft one too, but I guess it just crops up in my recommendations all the time.