Windows only: Free application Lunascape is a web browser capable of running any of the three major web rendering engines—Gecko (Firefox), WebKit (Safari/Chrome), and Trident (Internet Explorer). That means that each time you open a new tab or follow a link, you can tell Lunascape which engine you want to use to render the page. The idea, in theory, is that Lunascape gives you the benefits of each popular browser in one. For example, Lunascape gives you IE support for the few IE-only sites still out there and no-nonsense speed from WebKit or Gecko.
You can switch which engine is rendering the current tab at any time by right-clicking the tab and selecting the engine you want. The browser itself supports plug-ins, but from what I can tell it doesn't support just any Firefox extension, which is unfortunate. Despite the indisputably cool feature set (namely the whole three-browsers-in-one hook), Lunascape is a little unwieldy and very cluttered. To be fair, though, it's currently an alpha release, which means you should expect a lot of rough edges.
If you're a web developer and you want to quickly check how a site will look in each engine Lunascape might be a winner, but I can't picture very broad adoption of this kitchen sink browser as is. Lunascape is a free download, Windows only. If you give it a try, let's hear what you think in the comments.