I find I like to use RTM to count through my exercise routine: X sets of Y pushups, Z sets of A situps, etc.

I find it really convenient to save an email template (text document or something similar) which has all these routines already typed out and then just send it to myself so that I can do this right before I go out to do my workout. So my email would look something like:

20 pushups
20 pushups
20 pushups
20 pushups
50 situps
50 situps
50 situps
50 situps

Then I email it off to my RTM import email address with the subject as "Exercise" (the name of a list I keep) and seconds later it pops up in the list. As I complete each set I check it off and I don't get los trying to keep count of what I've done. It also lets me do sets in random order to keep it all a bit more fresh and interesting.