NetLingo - This is an online reference for Internet terminology. The word definitions are easy to understand and contain links to other terms.
Webopedia - This was my Basic Geek Dictionary when I was a knowledge base technical writer for a software company. The word definitions are short and pretty easy to understand. New terms are featured every now and then.
Pseudodictionary - This is generally a dictionary of slang words, but you might want to poke around this site because it also includes some words from Webspeak like "J slash K."
MSN Encarta - This site is quite new to me but I’m recommending it anyhow since it has an encyclopedia, a dictionary, a thesaurus and this Internet quiz.
You can also try these printed Internet lexicon: