Welcome to WebmasterWorld Guest from 188.8.131.52
..actually takes you to the front page of Webmasterworld.com as everything before the "@" is ignored. With some further obfuscation you could get:
Which is *still* a link to Webmasterworld.com.
IE de-obfuscates the URL, but it can be a useful think for emails, especially if you're playing a prank on someone. It's fairly easily detected by anti-spam tools though.
(Note to mods: I *can* link to webmasterworld.com, right?)
Spammers exploit this feature by filling up the space in front of the @ with irrelevant nonsense, in order to confuse their uninformed victims. Just ignore that nonsense, and the remainder will show you what's really going on.
If you are looking to get into a "Members" page with a password dialog box (standard security) you can type your username and password into the url separated by a ":" and leading into it with "@"
[grumpus:firstname.lastname@example.org...] would log me in to the member area here, if there was an area using standard password protected directories. (I don't think that's the case with this site, but you get the idea).
<added> Doh - bird beat me to it. :) </added>