[[http://4-ch.net/ 4channel]] would create a link with text "4channel" that goes to http://4-ch.net/. It seems easy enough to implement: replace [[ with <a href=", replace the space after URL with ">, and replace ]] with </a>. It's based on the Wikipedia/ED style of linking and it's a lot more convenient than BBCode. You could also use [ and ], but those are sometimes used in plaintext which could be an issue.