How can I get the Unknown Sources option on my Dell Streak? I want to install one application which is not in the Market. I know that if I want to do that I should switch on the "Unknown sources" option, but I don't have "Unknown sources" on my Dell Streak. What should I do to get this option?
Depending on your firmware version, you might be able to abuse the ability turn-on engineering mode. Taken from StreakSmart:

Dial *#*#307#*#* on your Streak. Other than a quick flash on the screen, it will look like nothing has happened.
Go to your app drawer/program menu and tap a new icon labeled EMList. If the icon is not there, turn off the Streak. It should be there when you turn it on again.
Scroll to the bottom and tap app_security.
Tap update unknown resource property to enable it.

This will be the only way without installing a custom rom (Google StreakDroid)