TheBigWhois. A database of recently queeried domains which showed a status of still being available.
Verio.com/WhoIs.net. This is the biggest database of expired domains. It is used by most compilers of expired domains lists. When you search the database, look for words with common beginnings and common endings, otherwise you will get lots of rubbish and only one or two gems. A search for domains ending with "ingly" or "ably", will give you a whole page of available one word domains (searching for domains ending in "ly" produces too many results to filter manually). Searches for domains containing specific words can also yield good results. They provide a "no frills" registration service for $19 per year.
DeletedDomains.com. The DeletedDomains search engine is excellent for finding domains that have just expired. If you catch the system at an update (around 5am EST) you will be able to catch domains as they drop. Searching for domains deleted in the last 24 hours turns up a lot of rubbish, albeit fresh rubbish, and occasional gems. It is worth trying to exclude some of the rubbish by searching for short domains or domains with common word endings ("er", "ing", etc).
DomainsBot.com. Another domain search engine. This one occasionally lags a couple of days behind the rest of the net and it takes a little patience to get results, but still can be useful. There are good features for checking availability and popularity of the domains with a single click. The service used to be free but now there is a charge. Unless things have improved dramatically since it was a free service, it is not worth the fee. Their basic registration service costs $13.50.