What are you worth?
You are unique
Before you read anything below, remember this. It's worth what someone will pay for it. Sounds obvious and applies to everything else too, but it's even more true for domain names. It's both a great asset and the cause of many disappointments. For instance, redtyger.co.uk is obviously quite important to me and if I ever forgot to keep up the registration payments and lost the domain I'd likely pay a lot to get it back, but to anyone else it's probably more or less worthless. On the other hand should it by chance be essential to someone else, they have no choice but to use something different or keep raising their offer. Domain names are of course unique and that is what will give a domain value far more than any other factor. You may find a very disappointing offer for a domain countered by an offer ten times that from someone who needs it more. Neither offer is more correct than the other. Having said that, there are some general trends worth taking notice of.
A domain that does what it says on the tin
Below are the top 20 sites by traffic as indicated by alexa.com as of today, 20th December 2006.
Of all of those, blogger.com and possibly myspace.com are the only two that give any reasonable indication about the content of the website that the domain belongs to. Browse the rest of the top sites listed on alexa.com and you can see that the same is true for much of the rest of the list. The key is not the description, it's the name.
Keep it short
While it's not true that you'll be losing value with every letter, it's important to have the fewer the better syllables or words. It's impossible to be exact, you should be able to judge for yourself what is short and snappy and what isn't.
Be a .com
Unless you're a specifically regional site using a regional domain (google.co.uk) or using a domain extension that fits your website's theme (wikipedia.org) then a .com is likely to be the order of the day. Certainly they are the most valuable, with .net and .org following with a reasonably substantial value. Some other major domains, such as .info and .biz are gaining value as the most popular domains begin to run out and the domain extensions become more mainstream.
Be a .com
Unless you're a specifically regional site using a regional domain (google.co.uk) or using a domain extension that fits your website's theme (wikipedia.org) then a .com is likely to be the order of the day. Certainly they are the most valuable, with .net and .org
Take the results with a pinch of salt, results vary hugely so even as a rough guide it's open to a lot of variation. Nevertheless, if your curiosity needs satisfying for free, here are a few sites that will give you a valuation via a short form. But remember most of these places are selling a service, so don't crack open the champagne quite so soon if the valuation is flattering.
For the purpose of comparison, I used each one to rate redtyger.com, this sites alias which is, in my opinion, fairly worthless. Where asked I rated it as not marketable, not indicative of content, fairly memorable and since it's not specific to anything at all, has a large potential market but of use to a small amount of people. The last two can be very difficult to answer.