Play board games worth thousands of dollars.... for $20.

You do need to be willing to learn Tabletop Simulator's commands/controls and understand that there is no A.I.

I'll probably be buying a good handful of the paid DLC just because its so cheap compaired to the physical editions... but even if you're never going to do any DLC there's thousands of games for free (once you own TTS) on the workshop. Some are broken or crap, of course, but tons are fantastic.

Best purchase I've ever made, at least by far the most valuable purchase I've ever made on steam (out of my 500+ games).

Since buying tabletop simulator, I've used it several times to "test" out or see the rules/components of a physical game that has caught my interest. TTS has been an invaluable tool in helping me decide if a physical copy of boardgame is actually going to be a good purchase or not.

2018 Update: This is a rehash of the above but with an actual example. Recently TTS helped me realize Kingdom Death: Monster, despite being a fantastic game that everyone should play, is not something that I would want to purchase. This saved me at least 450 bucks (that is a litteral number). Again I just cannot stress the amount of value I've gotten from TTS.

2021 Update:
I'm old now, and I have a son that likes stuff with zombies... so naturally you'd want to buy him the Zombicide board game. Well even a base game is nearly 100 bucks and actually doesn't come with much. To get a full "Season" you'd have to first wait for a kickstarter, then back it for about 200-250 dollars including shipping, then wait literally 2 years and there you've got one complete season, if you wanted more its years more of waiting and another 250 bucks.

With Tabletop Simulator, the total cost for two people to access everything from season 1, 2 and 3 is $50 (vs ~$750)... again... TTS is, by far, the most cost effective and valuable thing on Steam period full stop.