That's actually a pretty complex question to answer. A domain name is only available for an annual rental, so when one drops, it's technically the same as one that has never been registered. Of course a dropped name can have links still live on the net, and may be in a number of blacklists, but that's a different consideration.
You also need to differentiate between expired names and dropped names. An expired name is one that hasn't been renewed, and will have to go through a drop cycle of around 70 days before it becomes available again. During that period it may be renewed by it's owner, or it may be claimed by the registrar - in either case you don't get a chance to grab it.
You can register these names through any registrar when they become available. Lists of expired, dropping and dropped names can be seen on a number of sites.
www.deleteddomains.com and
www.whois.sc are two that are widely used.
You can also get some interesting facts about the keywords in the domain by using this site -
http://www.digitalpoint.com/tools/suggestion/