I've created a new plugin for keepass2 to allow challenge-response 2nd factor authentication. This is superior to OTP in certain scenarios; for example, it allows you to use multiple yubikeys to access the same database. You can find it at https://sourceforge.net/projects/keechallenge/. It's very much an alpha release right now, so be warned. That said, it runs well on my system. If there are any other developers out there who'd like to contribute (especially those with Mono experience) please let me know.
Very nice, thanks for creating this plugin!
I've added it to the KeePass plugins page: