Nihongo Zokugo Jisho
Do you know what KY means? Or are you an アラフォー? This is a very cool dictionary composed mainly of colloquialisms, old/obsolete words, and slang. It has a lot of very recent and new popular words and acronyms to search from, and the definition entries even try to pinpoint the year and origin of these kinds of words. The weekly "most searched" ranking is also pretty interesting.