if you are just "catching" dropping domains, how do you value which one is worth?? i think one is google PR, but what other things?? for example, if you see that "fontzazoo.com" (invented name, i don't know if it exists) is gonna be available, and it has a PR 0, but if you search for the word fontzazoo, it appears 10,000 times, is that something valuable to acquire the domain?? or what other things are important? thanks ;)

First of all I'd check the domain itself, if it contains interesting keywords, if it's short, if it's an easy to remember domain.

Then you could try to guess how much traffic the site had before expiring. There are many ways to do this: you can check google for backlinks to that domain, you can check alexa for their traffic rank and you can search on various search engine for other sites talking about the site that was hosted on that domains. Also Way Back Machine can help you understand what that domain was hosting.

One more very important thing to check when looking for expired domain names is if they're listed on dmoz and yahoo directory. If they're listed this could boost your PR immediatly.

Hope this was useful

mostly previous traffic makes the domain valuable also if its generic term like 'computer' or 'sport' etc.. thats attract so much type-in traffic

ther eis tools that show the backlinks pointing to a domain which also attract traffic