Quite a few HTML entities work, too: for instance, you can use < and > for "less than" and "greater than" symbols.
E.g. all of the following will create an identical link (including the bracketed domain, if the user's settings are configured to show it):
<a href="http://example.com/">http://example.com/</a>
<URL:http://example.com/>
http://example.com/
In that last case, only as long as the character immediately preceding "http" is a whitespace character (or none, at the beginning of the comment). An HTML tag immediately preceding is not considered a whitespace character (even if a whitespace character immediately preceded the tag), which is useful when you specifically don't want it to link; e.g. the trick I used for my 3rd example above (simplest case, a useless tag that is stripped out but prevents it from auto-linking):
<>http://www.example.com/
Putting the link in quotes, parentheses, <p></p> tags, etc. would also prevent it from auto-linking.