You basically have three to chose form:
1) Steel Fury which was made in 2007and has been greatly modded and expanded, Panzer Elite which goes back to 1999 but has been heavily modded and updated and is still played and T34 versus Tiger from 2007 which was almost sent to an early grave because it was buggy , unfinished and the developers dropping it like a hot potato. Currently only Zeewolf mods it but his mods are not free.
Steel Fury is only SP( here's the link to the forum
http://graviteam.com/forum/index.php?board=1.0 )
, Panzer Elite is both SP and MP ( here's the link to their forumhttp://pedg.yuku.com/directory#.U1CdaE2KCUk)
TvsT also but nobody plays the MP anymore and for the SP you need to subscribe to Zeewolf's membership there's sticky dedicated to it a little above.