I use Dropbox as well. It's free and allows you to use it on more than one computer and you can do it without having to have the other computer on the network turned on to access a shared drive.
You can also use multiple operating systems with this. I have it running on OSX, Vista and Linux.
With Dropbox, the Dropbox software creates a local folder on your drive on your various devices. Similar to what RGWilder posted for sharing a network drive, you can then create a Beersmith folder in that Dropbox folder and use the "Change Document Directory" command within Beersmith and use that Dropbox folder instead of the regular one. Beersmith will then save out all of its setting, recipes, inventory and everything else in that folder. Then when you open the software on another device, it opens with that latest info.
If, however, you have it open on 2 devices, the last one that you close will update those files and sync, overwriting the files. So, if for instance, you have it open on 2 devices and write notes in 2 recipes, those notes will only be saved from the one where you last closed the software. So, only have it open on one device at a time. It syncs quick, though. So, I sometimes have it open on my Mac and write something or tweak a recipe, close the software, go over to my laptop across the room and open the software and it's synced already. Pretty cool.