Windows 7 and various ditro's, sometimes Mint, sometimes Ubuntu, depends.
To be honest though, VM players like VMware Player, Virtual Box, WINE and so on, I don't really see the need for dual booting any-more as much. Creating VM's is much more easy and fun IMO. I guess if you're gaming though, you kinda need to dual boot really.
Make sure you use GRUB as your boot manager and I recommend having a Windows partition first.