Apart from expired domains, those that have been dropped with existing traffic and page rank are extremely valuable.
Using a service such as that of www.FreshDrop.net , you will be able to locate domain names with existing PR (Page Rank - ranging from PR2-PR6, although PR2 and PR3 are the easiest to find).
Page Rank (PR) simply indicates that the domain was receiving traffic prior to expiring or dropping, and of course, online merchants place a lot of value on purchasing a domain that has already generated traffic, saving them the initial work of launching a domain, or waiting for it to be aged enough to appear in the search engines, not to mention the fact that domains with page rank carry more weight in the search engines such as www.Google.com .
There is also discussion involving new domain names being placed in what is referred to as a 'sandbox', where they are kept for a period of time before being released to appear in search engines, another reason why aged domains are so valuable to merchants. At FreshDrop.net, you can browse through their search functions selecting to locate domains with specific PR.
Personally, I set the search tool to seek out domains with a PR of no less than 2. You can do this by clicking on the tab marked "EXP Name" and running your search query. Finding decent domain names with existing page rank isn't all that difficult and by featuring them within domain auctions on sites like NamePros.com, DNForum.com (paid membership) or Ebay.com can instantly generate a consistent income with little effort. Once you have structured your system and are seasoned with doing your daily rounds of searching through domain listings, dropped and expired domains, and checking sites like Fresh Drop for domains with PR, you will find it incredibly easy to generate dozens of valuable domain names that you can quickly flip for profit. It's an exciting business to be a part of, no doubt about it.