Both Amazon and Google, the two biggest players in the Android app space, have the ability to show “badges” so you can link people directly to your app. Maybe it doesn’t surprise you, but Google did it better for the Android Market.
Android Market button
UPDATE: see the most recent Google Play badge guidelines here. They change periodically:
https://play.google.com/intl/en_us/badges/
Google went so far as to have a badge generator for you, and to host the images. That’s pretty handy. You just enter your package name, select the button size, and you now have a snippet of code to paste into your page.
Overall, Google’s solution is pretty slick.