I would also recommend categories for what metadata is exposed; if messages are encrypted at rest on your device; cross platform ubiquity.
You should include bitmessage, and i2p-bote.
I am glad that this is only the first step but I do think that you shouldn't have done it alphabetically but rather by score and usability.
There's a weird case around iMessage and any tool that is provided by an OS vendor. I think we need to add a note about this, but in those cases that company could inject malware or a backdoor either in the messaging system or somewhere else in the OS. Since we're trying to tackle one hard problem at a time (secure messaging but not secure operating systems and software distribution) there should be an extra caveat about offerings from OS vendors.
The only tool that gives strong metadata protection right now is Pond, and we aren't listing unusable tools that aren't out of beta yet. We considered but haven't yet included bitmessage for the same reason.
The iMessage issue isn't related to iMessage being developed by the same company that handles the OS. It is that Apple holds the decryption keys for your messages; this is security by policy not security by design. http://blog.cryptographyengineering.com/2013/06/can-apple-re...
What fails the mud puddle test is data stored in iCloud backups.
If you don't use iCloud backups, say goodbye to your I messages because they are only stored on the device.