Simulators for WoW are nothing new -- Rawr, for instance, has been around for years, and is steadily snowballing into a one-stop shop for simulating all classes (it's not there yet, but I still love it). In case you're scratching your head at this point, a simulator is like a spreadsheet, but much smarter -- instead of using some general approximations to calculate how your gear is going to change your DPS, it basically goes ahead and plays a model version of the game for you. Edit: apparently Rawr is not a simulator -- it uses formulas that come up with the same answer every time, much like spreadsheets. We still love it anyway.
What is new about the simulator I want to talk about today, which seems to be entitled "Mr. Robot," is that it runs on the web, in Microsoft's Silverlight framework (Silverlight seems to have come about because someone at MS saw Flash and decided they wanted one too). This means it's cross-platform and there's nothing to install (well, except Silverlight, but you may have that already). They're only doing sims for Death Knights right now, but the team says more classes are coming (I hear Warlock is next, but don't quote me).