For those of you who went to Sam's talk on algebraic topology or have read about and not understood what cohomology is all about, theres a fine page here which explains it in all its gory detail:
http://simple.wikipedia.org/wiki/Cohomology

The integral page also has some amazingly insightful words of wisdom:
http://simple.wikipedia.org/wiki/Integral

Seriously though, what mathematics websites (barring wikipedia) do people regularly browse whether blogs of mathematicians or resources for a quick run down of something of interest?

well wolfram mathworld can be useful, its pretty similar to wikipedia.
On an unrelated note here's a sweet function calculator I used to use when I actually did analysis lol:
http://wims.unice.fr/wims/wims.cgi?module=tool/analysis/function.en

Like xkcd , I am slowly falling in love with Simple Wikipedia.

this one has some good stuff on algebraic topology, as well as some other algebra bits and pieces.

It's funny that you mention the Simple Wikipedia article on Cohomology; when the xkcd comic about Simple Wikipedia came out, it was the first article I checked to see if it existed. And it didn't! It was created really recently.

Now for some maths websites:
The n-category café and the n-cat lab
John Baez's webpage, especially This Week's Finds in Mathematical Physics, which contains many really interesting mathematics.
Ronald Brown's webpage, with many resources on higher dimensional group theory and homotopy.
The Grothendieck Circle, a comprehensive archive of everything Grothendieck (and awesome).
David Mond's webpage
The Casters' webpages: Eugenia Cheng and Simon Willerton.

Tim Gowers' blog
Terrence Tao's blog
The Everything Seminar
The Secret Blogging Seminar
Topological musings
The unapologetic mathematician
Ars Mathematica

Arxiv, e-print service with many mathematical articles, though not always of great quality (Cosmin can probably tell you how many proofs of the Riemann Hypothesis are available on that website).
JStor, lots of really good mathematics articles, you have access through the Warwick library website here, which also has several other good links such as MathSciNet and Intute.

That's all I can think of for the moment, I probably missed many really good websites.

That's a pretty extensive list, not sure how many good ones I can add. Possibly the Erdos number calculator and the Mathematics Genealogy Project, they're pretty useless though. I would have said Pro Theory, but apparently I need to cut down on the trolling.

I was arguably too harsh on you in the hustings. It was really stuff like the fake Colin accounts that I was moaning about, and that wasn't just you. Most of your trolling is now restricted to real life.

This one is pretty good is you're interested in the stuff to answer my own question (author of the Fractal Geometry book):
http://www.mcs.st-andrews.ac.uk/~kenneth/index.html

Slightly off topic, but thoroughly entertaining nonetheless...

http://gnomopedia.comli.com/wiki/index.php/Gnomopedia_Home_Page