For the last few months, I've been playing Rifts(R) for Savage Worlds on SavageRifts.com, a play-by-post site. This has been my first experience playing any RPG by post, and it's been an interesting experience. The GM has made an effort to streamline the mechanics of play - everyone rolls Notice at the beginning of each scene, and the Quick Combat rules are used for anything other than the most significant battles.
What I've noticed with play-by-post is that the roleplaying really comes front and center. This can be a refreshing change for players who like that aspect of the game, and perhaps a deal-breaker for the min-max-munchkin who wants to get in there and do 75 points of damage with his boom gun. As someone who had gone a long time without playing, I've found it enjoyable to make the transition, and relaxing not to be in charge of the story for a change.
SavageRifts.com has recently started running other settings as well; if you're interested in playing but don't have the time for a regular session, I'd definitely recommend it.