I use KeePass to keep track of my licenses. You can even set it up to fill in serial numbers just like passwords on web sites. Here's an example...
(I guess I didn't set this one up to automatically fill out the form, but you get the idea.)Software Database.
That is actually a valid license for EncSpot that anyone can use.
Here's another one that will
fill out the registration automatically...Software Database.
If the program has some kind of registration file or something (like AnyDVD), you can use the "Attachment" function on the bottom. So far I haven't ran into any licenses that wont work with KeePass.