I got my Swiftkey keyboard (closed beta) direct from Swiftkey. I have one other beta app that I am testing, but can't say what it is.
In both cases, I was given a link in an email, tapped that link and it downloaded directly to my downloads. Using the built in My Downloads
app I just tapped the download and it asked if I wanted to install. (I already had the setting for Unknown sources checked).
Most of the apps you will want will come from the Market, and you would be silly to download from anywhere else unless
you know exactly what you are getting. Remember, dodgy Chinese servers are the source of most malware. That ripped off
version of some market app isn't worth having your tablet strip mined.