Alternative market app without country limitations I want to download Apps from the US for researching purposes. A co-worker of mine could do that by installing an alternative Market. He did not have a rooted phone and probably got that app from the original market.

The icon of this app was similiar to the original market's one but blue instead of green as far as I can remember.
I think SlideMe is the best Android alternative market available right now. There's an app that you need to install in order to buy apps directly via your phone. 

There's also GetJar - a number of devs are releasing their products on GetJar before or along with releasing them on the Android Market.