I'm looking for resourses on undergraduate mathematics, explaining ideas behind well-known proofs, etc. What are some other sites like T. Tao's blog or T. Gowers'?
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John Baez's site has some stuff: http://math.ucr.edu/home/baez/
In particular, he was one of the first people posting things like this on the internet, way back in the Usenet days when he published a "column" on mathematical physics, which are collected here: http://math.ucr.edu/home/baez/twfshort.html
For a very fun free-wheeling discussion of (strange) ideas, give http://what-if.xkcd.com/ a try. That isn't quite what you asked for, but I'm confident more entertaining.