I've started working on my own private weight/fitness tracking site.

http://gym-bunny.herokuapp.com/weights

It's still early days -- if you click on the "weights" item, you can see that my weight timeline is hard-coded into the app (well, it's in the database).

What I really want to do is add workout information, sets, reps, and so on, and get the same charting capabilities that I have for weight. What's nice is that I built the weight graph as a reusable component, so as soon as I figure out the data schema for the exercise stuff, it should be as simple as adding another page to the app to display it.

Yes, I know that a lot of this functionality is replicated elsewhere, but no site (Livestrong, Fitocracy, etc.) is exactly what I want. Either they're trying too hard to sell you stuff, or they have a clunky interface, or something. So, since I'm a programmer, I figured I'd do it myself

To be added:

• Multi-user interface/login capabilities (right now it's all me, all the time, but it needs to support multiple users). I'm intimately familiar with how to do this, but it's a hassle in initial development, so I may just leave it single-user for now. Even if people on the internet trash my database on Heroku, I can just reset it by uploading my dev database.
  
• Add/edit/delete data points via AJAX
  
• Mobile testing! I'm using a clean, bootstrap responsive layout, but I haven't hit it on my phone/iPad yet; I need to make sure that everything looks good and performs well. I'm especially nervous to see if the charting engine (jqPlot) works on resource-constrained mobile devices.
  

Yes. I'm a geek.

Looks like a great site, keep us posted. I agree with you that most apps and websites that already do this, lack in different adpects. Its great that you can do something about it! Good luck.