I like the +badges, which is such a trivial thing but is good fun and positivity. (I may beg some day for someone to post this code somewhere so more games can use it, or may try a wiki/game hybrid thing with this idea.)
I'm using a lot of incremental XP awards in some of the stuff I've been kicking around for a few years, but also looking at some other things that are a bit more ephemeral. I know @Sunny had some ideas that were XP-linked or similar that were used for something other than buying stats (but could be put toward special goals) that I hope she'll chime in with here as well.
Something I think might be really nice? Something that stores the +reccs (or whatever system a game is using along these lines) someone has rec'd by others, and can display them -- the text of them. This may be, code-wise, too huge a bear to do, but being about to +recc/list <name> and see what good things others have said and given thanks for is something I know I want to figure out how to do and would love to see more of on games in general. A lot of times, people who put a lot of effort into things do so somewhat invisibly -- or they feel these efforts are not really even known about, seen, or appreciated. Being able to list one's own would help to counter the latter a little, and this would also make it possible for others to see: hey, this is a player who regularly goes out of their way for others, that's really cool!
Recognition for someone's contributions, no matter who they are or how big or small that contribution is, is something I'm big on.
Cautions: I know that some of the things I'm looking at, I still need to hyper-scrutinize in terms of scale. Since I'm gonna guess you're asking for things to implement on FC, scale's a huge factor as is a game just starting vs. a game that's been running for a couple of years. Something that'd be great on a game from day one might be easy to implement if you do it later on a small game with a dozen players and mayeb 30 total characters, but it'll be harder to tuck in to a game with 20-35 players with 2-3 chars each and starts to take a lot of work and time. (Reno1 was like this, with the 1E -> 2E werewolf conversion. Ask me about my eyetic! Or don't, please? It's embarrassing.
) Similarly, some stuff will be easy on a small game where everyone can talk to staff about some custom thing or another in depth, and cook up something unique for them. Some things are just too big to really do past a certain point without it becoming more trouble than it's worth.