Arena is a free chess client (http://www.playwitharena.com). Free as in free beer, not open source. However, it is really quite impressive, and it runs flawlessly under wine which is a bonus since we don't use Windows at home any longer. I'm not very good at chess, so the race to get more and more powerful engines leaves me bemused. However, there is a version of Crafty in which someone has spent a lot of time thinking about smart ways to dumb down a computer engine.
You want Crafty SE. I got it from here: http://wbec-ridderkerk.nl/html/details/Crafty.html ; scroll down and download this link
Here is a "special" Crafty SE with personalities, modified/compiled by Mike Byrne:
Crafty 19.17 SE(SSDF)
Put it into the Arena engines folder and install it as usual with the install engine wizard.
To choose a personality, take advantage of the Engines -> Manage -> Special tab; look for the field called Init string
Here you can choose from a range of predefined personalities (you will see text files with all the names in the Craft SE directory you unzipped).
You can simply enter
into the Init string
To see what it does, bring up the engine "debug" window (F4). You will notice that the crafty engine is so kind as to specify exactly what the personality does. Personalities can be handicapped by the use of the opening book, piece values etc. The SEversion also allows randomisation of search depth, and other smart ways to dumb down the machine. It is funny that the real test of AI for computer chess is emulating the strengths and weaknesses of human intelligence.