I'd like to try out some Android beta software I found online, but when I tried installing it on the device I got a message that said that installing non-market apps has been disabled for security reasons. Can I bypass/disable this restriction? I am using the at&t Captivate (Samsung Galaxy S) with Android 2.1.
No need to load a custom ROM. You have two paths possible -- You can simply use special software on your PC to sideload apps or you can do some edits to the configuration databases on the phone to bypass the block permanently.
This page covers both methods -- http://samsungcaptivate.wikia.com/wiki/How_to_Sideload
If don't want to root you can use http://www.androidcentral.com/sideload-android-apps-all-you-want-sideload-wonder-machine Its much easier than side loading with adb
The first step is to root your device: http://forum.xda-developers.com/showthread.php?t=739304
Then you'll want to try out some custom ROMs. That thread recommends using ROM Manager from the Market. I would start there.