I haven't bothered going through the thread looking for actual suggestions, since it seems to have swiftly devolved into animals doing hilarious and/or awesome and/or adorable things, so I don't know if these have already been suggested, but:
* I believe the BattleTech basic rules are available as free PDF on classicbattletech, although you would probably require a big hex map of some description for that. Models are optional, and fairly cheap anyway.
* A lot of Games Workshop Specialist Games stuff is now available for free online, although with that, models are quite definitely not optional if you're wanting to play in any sort of official GW store. A lot of that is smaller scale encounters though. Necromunda and Mordheim are still pretty sweet on the small-squad tactics however. There's also stuff like Battlefleet Gothic, which is pretty good for SPESS BATTLE, Gorkamorka, which is Mad Max With Orks, and Blood Bowl, which is not techically a wargame, but is still awesome.