Developers are really into Roman Nurik's new live wallpaper app, Muzei. A ton of extensions are already available in Google Play, some of which have neat tricks up their sleeves (see part 1 here). Here's the best stuff we've found so far in no particular order.
One of the most powerful tools in all of the modern desktop browsers is the ability to add extensions. Extensions allow the browser to do things that it can't do on its own - everything from simple tasks to advanced options. When it comes to desktop extensions, the limits are generally left up to the mind of the creator.
Up to this point, extensions haven't been all that widely adopted in the mobile world.
One of the biggest restrictions that we face as Android users is "device incompatibility" issues in the Market, even though the app in question may work perfectly on our device. For example, according to the Android Market, Plants Vs. Zombies is "incompatible" with my Galaxy Tab 10.1; however, when it was initially released to the Amazon Appstore (and before Popcap was bought by EA), I always played it on the Tab.