Professor Lana Trick of the University of Guelph, Ontario, Canada, studies attention and working memory. One of her research is on object tracking.
See how good you are at tracking multiple objects with this fun (but very hard!) Flash game: http://www.psychology.uoguelph.ca/faculty/trick/mot.html - Thanks Wayne B.!
Consistently got 2/4 on Difficult and perfect on the easier ones. It's a little annoying though when 3 or 4 converge. Makes you momentarily look there to see which is the one you were trying to track... then you lose one of the other ones.