This is the kind of site I'd been passively hoping for for years, but never really found until makeuseof.com recently featured AcronymFinder, who claims to be celebrating 10 years online (!) How I never found them, I'll never know.
With the ubiquitousness of the internet forging the way for new forms of slang and colloquialisms, oftentimes it's hard to keep up, especialy when it comes to acronyms. For example today used the site to look up IRL, which apparently means "in real life," aka, not online. Recent acronyms I'd also looked up elsewhere online, mainly using urban dictionary, were FTW(for the win) and RTFM (read the f****** manual).
AcronymFinder sorts possible definitions by relevancy as well as category. They take definitions from user submissions, and they have strict guidelines to ensure accuracy as well as relevancy. Regardless of this, they seem to have an extensive database that really help you navigate your way not only online, but in other jargon-heavy environments as well. Highly recommended to add to your keyword searches on Firefox.