Less relevant (unless you're on the free Google Apps plan, or want to do private apps outside a domain) now that this is available, but using Dropbox/GDrive for

Less relevant (unless you're on the free Google Apps plan, or want to do private apps outside a domain) now that this is available, but using Dropbox/GDrive for private app distribution is far less painful than email. (I imagine it wouldn't be too difficult to build a pseudo-Market over the GDrive APIs for such uses. Might look into that at some point.)