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).
It's pretty nifty, and putting a simulator on the web is a great idea. Loading your character from the armory is a breeze, as is configuring what buffs to test with and customizing your gear. It has a great display of how much it thinks any given piece of new gear will improve your character. And for the very-beta state it's currently in, it's a great piece of software (it was developed in a hurry for a Silverlight competition).
That's not to say, of course, that I don't have some problems with it. The UI takes some getting used to, but it's actually surprisingly usable. A bigger issue is that I don't really trust the numbers it's putting out. My DK may not be the most geared in the world (my raid tends to need my tank or healer more), but I'm pretty sure he puts out more than 2.9k DPS self-buffed.
One source of error may lie in the rotation system. Correct me if I'm wrong, people who know how the site works, but it appears that Mr. Robot literally uses a rotation to choose what abilities your character uses in the simulator. This is pretty old-fashioned - at this point, the theorycrafting for most classes uses a priority list to choose abilities, not a fixed rotation. However, Mr. Robot does get points for having a cool rotation editor.
By the way, the app itself is not going to be open-source (like Rawr is), but Team Robot promises to make the formulas involved "open to the community for debate," so we can tear them apart in best Elitist Jerks tradition.
I also need the app to output stat weightings for it to be very useful to me. Here's my gear theorycrafting workflow: load up my character in a simulator or spreadsheet, configure if necessary, get stat weightings, import them into Pawn. Now I can quickly see in game whether a given piece of gear is an upgrade for me, and if so, how much. Before you knowledgeable people start glaring at me, I know stat weightings change as your gear changes. That's why I generate new weightings every time I get new gear. It's still easier than tabbing out of game to compare every item I come across.
Overall, though, it's a very cool project, and when it starts to mature, it may provide some worthy competition in the WoW theorycrafting space.