What are some good maths websites ?

Here; http://mathforlove.com/blog/

In the aid of full disclosure, I need to tell you that Dan Finkel is also my nephew. But he is a brilliant young mathematician and wonderful teacher; he is also a polymath extraordinaire.

For general math stuff I use wolfram alpha

Math Plus Magazine
NRich
Khan Academy

my Web site, featuring what may be the only (not without good reason) online interactive relativistic velocity addition slide rule

Purplemath.com

I know, purplemath as @Dutchess_III pointed out.
And, you can look in Wolfram Mathworld too.

Reading auto/biographies of math scientists can be interesting too. To further your knowledge, you may want to read these books.
A History of Pi by Petr Beckmann.
e The Story of a Number by Eli Maor.

http://mathworld.wolfram.com/

www.wolframalpha.com/