Ubuntu Linux. You can use aptitude or apt-get to install pretty much whatever you want, it will run on whatever hardware you have at the moment, and it's free of charge.
If you already have a Mac you could decide to stick with the Mac OS for development instead (I use it; it works fine), though you will almost certainly not be running Mac OS on your server. So you'll have to figure out Linux anyway sooner or later. The Slicehost tutorials make it much easier -- a lot of the grunt work has been figured out and documented.
How does Ubuntu compared to Debian on the server side, these days? Will the new FreeBSD kernel support get moved to Ubuntu? The admins I know doing this for big installs swear by FreeBSD. (I've only managed my own desktop for quite a few years and have no clue. No O/S flamewar, plz! :-) )
Edit: I went Ubuntu-only on my desk a few years ago, previously Debian-only (well, one computer still have a Debian partition too and I also have a Mac). I'm asking about servers.
If you already have a Mac you could decide to stick with the Mac OS for development instead (I use it; it works fine), though you will almost certainly not be running Mac OS on your server. So you'll have to figure out Linux anyway sooner or later. The Slicehost tutorials make it much easier -- a lot of the grunt work has been figured out and documented.