Android: With more than 30,000 apps in the Android market, finding good apps and filtering the cruft can be challenging. AppBrain manages your apps online, recommends good stuff based on your installs and makes it easy to share apps with friends.
AppBrain fills a few niches in the Android app experience. The primary benefit is that, once you've installed AppBrain on your phone, you can install and remove apps from your phone at will from AppBrain's website, with the changes taking effect once you sync AppBrain on your phone.
It's certainly not an iTunes knock-off, but AppBrain does a decent job of sorting and searching Android apps and keeping the steady stream of barely-thought-out, poorly reviewed apps away from your results. If nothing else, we'd hope someone at Google notices how much more convenient it is to search and click to install Android apps on the web than through the Market on even the largest of Android screens. Android phones are all about syncing, so syncing apps that you'll probably get some use from is a handy service, indeed.
AppBrain is a free download for Android phones only.