http://xkcd.com/189/Although exercise has its own natural motivators - endorphins, looking better, feeling better, living longer, avoiding disease, improving your ability at just about everything - these rather petty things pale into insignificance next to important things like experience points and levelling up.
Thankfully, a couple of dudes realised this and made this website:
http://www.fitocracy.comBasically, you log your workout in the site and you get points in return. Different exercises get different points, and various metrics like weight lifted, distance ran, etc, affect the amount of points you get.
The gameplay is still a little bare-bones, and a large amount of the community interaction consists of people sending each other pictures of themselves in their underwear and frantically eye-humping each other, but it's still fun, and everyone on there is really friendly: If you have breasts or visible abs, really REALLY friendly.
The adonises are only a small part though and there's a lot of average dudes on there. I'm on there as mcdumer, so if you want to add me and laugh at my pitiful lifts, feel free.
I've had an on-again-off-again commitment to the gym for years, and I'm finding Fitocracy pretty good at keeping myself motivated. Maybe it's a feeling of accountability, maybe it's just a Pavlovian reaction after a lifetime of playing video games. Whatever the case, it works for me.