I created an Excel spreadsheet a while ago that people can download and use to have a Biggest Loser Competition at work, church, home, etc. The "bad" news is you either have one person acting as the Manager of the competition (which can be a hassle asking everyone for their weigh in information), or you let everyone enter their own information each week (for example at a job where the Excel file was stored on a shared network drive), and now everyone can see each other's weight. With the success of the biggest loser, I knew their had to be other people like myself who had taken upon themselves to make having a Biggest Loser contest at work easier. There are, but before we get into that let me state a couple of things.
1. In the end if someone really wants to cheat, they will (you could always strap your fitbit to your dog...). Cheaters never win... in the long run. 2. There are more sites than these, but some of them were obviously just a tool to splash TONS of advertisements in front of you. I have one of those listed in this review, and I'll tell you when we get there. 3. Each has its own charm with different features.
After looking at these I created some competitions.
1. Skinnyo.com is completely free. It only has room for 16 people so hurry.
2. Thintopia.com is asking for $5 per person (if you want to play for free you can, but the prize money will go to the highest percentage of weight loss who paid).Check it out.
3. If you have a tracking device (like a fitbit, or bodymedia), we have a $5 entry fee at the earndit.com site. This one should be interesting. The person with the most points wins. Sign up
Now let's take a look at some of the sites:
Fitocracy is the game you play to improve your fitness. Play it to track your progress, compete against your friends, and get real results in your physical fitness. They are working on having a droid app (there is a mobile site). It's free, but you can be a "hero" for $5 a month to help support the site.  You can create a group, and the look at the leaders in your group for a certain period of time. The focus of fitocracy is exercise. You can track your workout stats (reps, weight, etc). You can track cardio, and you can create routines that you repeat. Depending on what you do, you earn points. I was playing with it Internet Explorer 8 and it seemed to have some formatting issues. When I tried it on my phone, I had some issues (droid). Looks cool, and has potential. It just needs a little touch up. I will definitely check it out in a few months. It is in Beta, and you need an invite to play (contact me, or use the link above). Lots of potential here, especially for those gym rats. Even if this wasn't a competition, having a web site that can track your exercise for free is cool.
 In the past I've used Weight Loss Wars. It is a website where you can join a challenge, or start your own. It costs $10 to participate, but it does allthe calculations for you. You can even create a "private" challenge (for say your church group, work group, etc).  The competition only shows the change in weight, and you can choose to hide your weight in your profile (or only reveal it to friends). Visiting this site you will see a fair amount of ads. The site itself is somewhat "ho-hum" on the looks side but it does the job. There are blogs, journals, and other tools there as well. You can have it set to have a cash prize, but you would handle that outside of their site (using something like paypal).
Skinnyo.com is also free to use (unless you want a private competition where you select who is participating which costs $10). Skinnyo.com limits you to 16 people (either playing against each other or you can have up to 8 teams).  Skinnyo only shows your weight differences to other users not your weight. If don't have a giant group, and want to have teams this is a cool option. The way they calculate challenge winners is by the total weight lost during the challenge (which I think means percentage of weight