Claims to be the largest database of acronyms and abbreviations on the web with over 471000 acronyms. Acronymfinder.com appears to have more definitions (it claims to have 2,020,000) and sorts the definitions into categories, for example Science and Medicine, Slang and Chat. Acronyma returned fewer results with my test searches but as well as English, it also offers definitions of Spanish, French, German, Dutch, Italian and Portuguese acronyms.