Any compressor will do, despite what marketing literature will tell you regarding "bus" compressors.
If you want to try one that's specifically designed for master bus compression, and that also happens to be FREE, then grab
Kotelnikov from TDR. There's a paid version that gives you more flexibility with the sidechain filter, but it's pretty cheap IIRC.
If you have Platinum, it includes an emulation of a classic bus compressor as a ProChannel module. It's pretty good.