A few years ago when I was looking to slam down the debt I had accumulated, I ended up creating a spreadsheet and charting everything I owed (car, credit cards, student loans, etc). While the initial data point was intimidating, I made it my goal to make the line go down as much and as quickly as possible. That simple spreadsheet motivated me to cut my spending and use the extra money to decimate my debt.
Fitocracy.com is the fitness analog of my debt spreadsheet.
Fitocracy uses a point system. You log your exercises and earn points. After you've accumulated enough points you "level up" – much as you would if you were playing a game like World of Warcraft/Dungeons & Dragons/etc.
There is also a heavy social aspect to the site. You can join groups of people with similar interests or goals, you can follow your friends and keep track of their progress, you can even comment on and congratulate people for certain workouts or for reaching goals. So if the gamification (yes, it's a real word) isn't a big enough hook to keep you motivated, you can also get your friends behind you to keep you from slacking off.
Fitocracy is currently in closed beta right now, so if you want to try it, you'll need an invite code – which I just happen to have a few of :).
If you'd like to join and follow me you can use this link (only 9 invites left at the time I wrote this):
http://ftcy.co/tEziaJ my invites have run out – try one of the groups below.
If you're a fan of the Penny Arcade webcomic, this invite link will put you in their group: http://ftcy.co/uXhMAR
There's also a "Getting Fit for the Zombie Apocalypse" group that I joined: http://ftcy.co/vSvdgj