You can get NXC with the Bricx Command Center, which includes help files, macros, utilities, etc.http://bricxcc.sourceforge.net/
I looked at all the programs but did not like any of them. BricxCC seemed far too complicated but i was stuck in the house over the festive season and decided to try it (Iwasboredootmaheid). I have done a little bit of programming before but not using C so I was surprised how easy it was to pick up. I started by converting a program I had already written in NXT-G so I knew the logic, all I had to do was find the equivalent commands in NXC.
Just wish I had started earlier! Plenty of code examples on the 'web to follow and plenty of help on this site as well.
RobotC is a commercial program available here :-http://www.robotc.net/
I have tried twice to download and run it without success but I was happy enough with NXC anyway.