Normally, to sideload (install from outside the official market) an Android app, you need to enable the "Unknown sources" option in Settings > Applications, then copy the app anywhere on the SD Card, and install it via a file manager, such as Astro.
Unless, of course, a certain entity removes the whole option altogether, making it impossible to install off-Market apps unless you root. Aria, Backflip, Captivate, X10 all have this problem. There was a workaround with HTC Sync that "mistakenly" allowed sideloading, but that hole was patched by HTC shortly after.
However, there is light at the end of the tunnel. Today, AndroidCentral announced an open-source program that allows you to sideload apps, even to phones that don't allow it. The only requirement is that you enable USB debugging in Settings > Applications > Development.
So now, whether you win an app in a contest, a developer sends you a beta version, or you want an app that is not available on the Market, you will no longer sadly say: "Oh, I can't do that - I have AT&T."
Check it out in action:
Very cool – props to AndroidCentral.