Below are four (count 'em, 4) sites that offer free brain tests for anyone who cares to know their cognitive profile. The offerings go from working memory to auditory processing to visual acuity.
The links below, for instance, are different tests for Sustained Attention, something ADHD kids are supposed to be bad at.
Psytest KiTAP: http://www.psytest.net/index.php?page=KiTAP
Some of these sites have paid sections, but the ability to get your own baseline tests done (and even compare results from one to another) is very useful. If you're into that kind of thing.