It's not a real tag anymore, but <xmp> was really cool. Everything between the start and end tag was presented verbatim, with no HTML parsing. Basically a lazy man's HTML escape in a fixed width font--good for pasting in code. We use it every now and then for our internal tools since Firefox is down with xmp.
http://www.blooberry.com/indexdot/html/tagpages/x/xmp.htm