Mac: doubleTwist, the media manager that syncs all the devices that iTunes doesn't, has thrown its own Android search interface into its Mac client. It's also opened up both its Android and substantial podcast search to the web as a whole.
The Mac update is expected to show up later today, with a Windows update to follow, but it's using the same web backend that doubleTwist has opened up to the public. It's a nicely pared-down, ad-free experience for searching the Android Market, and all the usual ratings, reviews and screenshots are present. You can't, however, directly install an application to your Android phone from doubleTwist. It's a restriction common to all apps that sync Android devices, and it's one being worked on. In the meantime, you can use a syncing system like the previously mentioned AppBrain to quickly install apps from the web and have them appear on your phone.
doubleTwist has also opened up their podcast search on the web, with a pretty big database of shows, audio and video playing of the latest episodes, and links to subscribe in both doubleTwist and through standard RSS means. If this little media syncer that could gave its playing and organisation interfaces a bit more polish, you could really start calling it an iTunes alternative. It's a free download for Windows and Mac systems.