Ubuntu is a good one. Had moved to Mepis when I bought my Mac. It is an Ubuntu based version with a little extra thrown in. The Mepis founder (Warren) uses a Mac Pro as his primary system. They have a pretty nice forum also, though not nearly the user base of Ubuntu. Think the next version will be moving to Debian based.
Another pretty nice one to start out is PCLinuxOS.
Linux Mint - another Ubuntu based distro, has many more media codecs installed to work with your audio, DVD, etc. out of the box. It can make that side of things a little easier first time out.
You can check out distrowatch.com and read through reviews for maybe the top 5 or 10 distros. That'll help in giving you a good overview for some of the differences between a few of them.
A lot of people like openSUSE also. But have never tried the open version myself.
If you like to do a lot of system tweaking, you may like a KDE distro better, such as Kubuntu or Mepis.