Where can I find databases of tactical problems, categorized and ranked by difficulty? I'm toying with the idea of building a tactical training tool. For my idea to work, I need positions that are categorized by motif and difficulty.
A lot of tools seem to have this, and I doubt they're all building up thousands of positions by hand. So there must be databases out there, available for use (free or commercial are both okay), and probably already tagged. Any idea where to start looking?
This kind of thing isn't done by hand, but by computer programs. It is not too difficult to run some program on a game database and extract all positions suitable for a tactical exercise.
Sorting them by difficulty will be done over time, when statistics accumulate how often they are solved correctly.

Of course there are tactical exercise positions out there, but I don't know any that are already sorted by motif or difficulty. This is a site with 10,000 tactical positions (surely mined by a computer):

http://www.wtharvey.com