If you haven't heard yet, Mozilla is working on an awesome new phone OS called Firefox OS.  From a developer's perspective, highlights include:

While it's now possible to create your own FirefoxOS build, many of you probably don't want to take the time quite yet.  No problem -- there's a faster solution for getting a sneak peak at Firefox OS:  Firefox OS Simulator (formerly R2D2B2G). Firefox OS Simulator is installed into Firefox like any other plugin, and once activated, is activated by selecting Tools > Web Developer > FirefoxOS Simulator.

Once installed, Firefox OS Simulator allows you to move from screen to screen, open any app (though some APIs like Camera don't work within the simulator yet), install apps from the Firefox Marketplace or any other domain, and just about anything else. FirefoxOS Simulator is great for developers looking to do heavy Firefox OS app development or simply get a preview of what's to come from Firefox OS.  Grab Firefox OS Simulator and get excited about Mozilla's upcoming platform!