Forum Moderators: buckworks
traffic stats
domain name
the website itself (content, scripting, etc)
active membership (for forums, online services, etc)
inbound links
Having at least 6 months of revenue and traffic stats can help a whole bunch. For sites with ad-based revenue the general rule of thumb tends to be 12-24 months of profit, depending on how much traffic/revenue history you can provide, site maintenance involved, etc. Subscription based sites follow a similar formula, though I have no firsthand experience selling a subscription-based site.
You may want to stop by dnforum, or a similar website sales forum, and just read through the different developed websites for sale. Doing that for a few months gave me a good idea of how to evaluate a site's worth.
If your site possess something very valuable, but you just haven't found a way to capitalize it. The value depends on who you sell to. Say if you have a "legal and sellable" database of high income online users with their physical address and phone numbers...etc, Ogilvy will probably pay you an arm and a leg for it. The value varies greatly depending on how much money the new "buyer" can make from it.