Oh, comeon, Philipp... as if we all don't use Firefox, and type ^K instead of go+gle... :-)

Domains Google needs to buy:

ggle.com/
g000gle.com/
www-google.com/
goooogle.com/
gugle.com/
joogle.com/
googl.com
googol.com/ (wtf how did domain squatters get this one? I know it used to be a legit page...)
googol.net/
googol.org/
gmail.net/
gmail.org/
googl3.com/
lgoogle.com/
gmale.com

Maybe some were domains that other people registered, and Google used udrp to get back.

I'm not savvy in this area, so take my guess with a grain of salt.

Google buys domains like that because if someone screws up in typing the name, or someone who has never been to Google before, they can still find their way to Google without having to search for it. You can't search for the best search engine until you get to the best search engine ;P