I’ve been using PlusPlus across a bunch of Slack channels — betaworks, Digg, Instapaper, etc. — and it’s really fun. It uses only two simple actions (plus karma & minus karma), but with enough users they pack a lot of (mildly spiked) punch. PlusPlus more or less immediately became central to the culture around here. And it only took a team of 73 senior devs a year to build. [And, right, disclosure: I work at betaworks.]
BONUS PROTIP: One of the foregoing sentences may not be entirely true.