Windows only: We're still waiting for serious extensions to hit the still-young Google Chrome browser, but if you're dying for more advanced Chrome functionality—like mouse gestures, link dragging, IE tabs, etc.—a tweaked Chrome build called ChromePlus is worth a look.
Most of the extra features in ChromePlus are available in some form or another on other browsers. The IE Tab functionality mimics the IE Tab extension for Firefox, which renders an Internet Explorer window inside Firefox (or in this case, Chrome) to give you access to those IE-only sites without leaving your preferred browser; the mouse gestures feature mimics mouse gestures available in some form or another on, well, virtually any browser (except you can't add any custom gestures yet); and the Super Drag feature lets you drag a link to open it in a foreground or background tab. (See the options dialog below.)
(Click the image for a closer look.)
It's not all that much considering all the potential you get from full-on add-ons (which are already beginning to find their way to Chrome), but it's a decent feature boost if you've been wanting any of those particular tools added to Chrome.
ChromePlus is freeware, Windows only. (Note: You may have some trouble with ChromePlus on a 64-bit install of Windows 7—I had trouble getting the app to work correctly on my install, and so did one of the folks at Download Squad.)