I think a lot of people use My Fitness Pal - it's very feature-full - for example, it integrates with facebook so if you're exercising/dieting as a group, you can share details and encourage each other - my son has been doing that (as a non-diabetic) as he tries (actually succeeds very well) to lose weight and get fitter.
I haven't been able to get on with it myself, but do use one called On Track from Medivo which I personally like very much, having tried several. Your requirements will be very personal to you and you'll need to find one that just fits the bill for you and what you want from it.