The sound of MG Elkins opening a can of worms ... tiptoes quietly away. . .
-Vassal is a PBEM platform (it might also involve the ability to play live) boardgames- (well board wargames for sure, not sure if others have been adapted for it -but I imagine pretty much anything might work there). It is (I think) open source software- or at any rate, it is a free download - however what can limit game title's availability is copyright/ publisher propriety things ( for instance the boardgame company that I work with hasn't gone to Vassal yet ... but other companies, like GMT Games have and their Vassal mods are freely available on the gmtgames.com site.
Before anyone asks (and I have been asked before), I don't do Vassal myself- have not had the time to learn the interface - I did mess around with Cyberboard back in the day- but never had the time for that either ... unfortunate I guess since I was planning on playing GMT's Eastern Front series on that platform ( maybe some day. ).
... I remember when I was naive enough to join an mmpog that claimed it was the ultimate wargame (spoiler- it wasn't...) - I was responsible for interviewing candidates applying for my group- and one of the questions I had was if so and so had experience with wargames - I got a lot of answers that I was not really ready for... chess, Axis and Allies, Stratego.... I was expecting things like Squad Leader, ASL, the titles David mentioned (or the WWII equivalents)... and after having been a member at BGG for some time well, it has only really underlined that situation.