It is impossible to overstate quite how much open source emulator DOSBox has done for PC gaming. Without the tireless efforts of its volunteer staff, vast tracts of PC gaming history would be all but lost to us and our fancy modern operating systems. Yeah, the likes of Steam and GoG may be keeping olden classics in print, but c'mon - they're doing it for the money more than for the love. The guys, though - they're bally heroes. Even those aforementioned download services occasionally hook their wares onto DOSBox to ensure maxi-compatibility. So, on behalf of the PC gaming community, I would like to award them the biggest invisible medal in the world for it. Perhaps more usefully, I can direct you to the DOSBox website, where a brand new version of this essential retro gaming tool has just become available.
There aren't any really mammoth, sweeping changes (so you're stuck with typing in command lines, unless you install one of the various GUI frontends doing the rounds), but there is a lot of under the hood tweakage to improve speed, stability and Vista support. Some of the games that previously proved beyond its miracle magic may now be supported, so it's well worth checking in case you do have some old favourite you feared would be shelved forever. Also improved is the sometimes hit'n'miss CD-ROM detection, which should make life a little easier for some games. Oh, and there's some new rendering modes if you're into that sort of thing. Me, I'm a bit of a purist and don't like graphics tweaks that noticeably alter the original look of a game, but some folk do love their smooth sprite edges.
It is, as it always has been and hopefully always shall be, available for free from DOSbox.com. Those handy frontends are also available from the download page, including the wonderfully-named (but sadly OSX-only) SirDOSalot - a title I've lately been adopting in various online games.
Thanks, DOSbox. ThOSbox. If you really want to show your thanks, you really should drop 'em a donation - see the button top left of their front page. On an absolutely unrelated note, there's also a very nice button underneath our own search box. Cough.