Used to obfuscate links on forums and imageboards (such as 4chan) to either circumvent link blocking or to prevent the links from being indexed by search engines such as Google. Also helps deter lamers, bots and newfags from hoarding files from sites which have limited bandwidth.

The hxxp:// prefix is often used in hacked password lists for pornsites to help prevent them from being deactivated.
by Burningmace October 26, 2007
Also used to obfuscate links sent as NSFW (not safe for work) - so that you don't accidentally click on it and get into trouble. It gives you the need to copy it down and change it to "http".
by flaming grunt April 5, 2009