Freebird is by far the best I've seen, and it's only about £20 a year, including updates 1-2 times a week by email. I have used this for three years and highly recommend it to anyone. The guys who run it are incredibly helpful and are always open to suggestions to improve it. It is totally non-profit, any money raised above the cost of distribution goes to a charity (nominated by the members).
It's in Access format, and is the most comprehensive database I've found, covering loads of types with histories etc, and is very easy to use.
Go to http://www.freebirddb.com/default.htm.
One thing to watch out for - with the expensive ones if you don't re-subscribe you run the risk of being locked out of your sightings until you pay again. I had this with the del-boys known as "Aviation Databases" - after your subscription ends you can't even access the data you've already paid for. Avoid them with a bargepole!