I want to develop an app using Adobe AIR. But I have to sign it using a code signing certificate. I don't wan to buy a code signing certificate. Would it be OK if I distribute my app with a self-signed certificate?
The only difference between using a real certificate and a self-signed certificate is what the user sees in the initial installation dialog. With a real certificated they'll see a yellow "!" and the app will shown to be of "KNOWN" origin, and your company name will be shown. With a self-signed cert, there will be a red "?", and it will say the app's publisher is "UNKNOWN". You can see samples of the two dialogs at the very bottom of this page.
So realistically, it comes down to whether you're okay with people seeing a scary warning at install time. If you're only offering up the apps as a "use at your own risk" thing, or the app will be used mainly by a small group of people who already know who you are (an internal company app, e.g.) that may not be an issue, but if you hope for random internet people to come use your app and trust it, a cert may be a good idea.