I'm curious. Which useful extensions are you talking about and why are they not listed in webstore? The one extension that I mentioned has two versions. The non-webstore version doesn't abide to Google's T&C [lets you download Youtube videos].
Some devs don't bother to put their extensions in the store, for instance this extension to play music files embedded in image files on 4chan: http://dnsev.github.io/4cs/
