The main way I log my weight is on FitDay (PC version, so I can't link to it), but I recently decided to try MedHelp to see what it was like. I really like it, and it's free. You can see a wider range of data at once, marking off events such as exercise, parties or your period, and you can see graphs for weight, measurements, BMI, and calorie consumption. Mine is here
, and also the link in my signature, if anyone's curious. There are also trackers for pretty much anything imaginable, but I generally use a spreadsheet I created myself for that stuff.
I'd be interested to see anyone else's MedHelp tracker, if they feel like sharing it.