Thugie made this plugin for me but he is not developing it anymore becouse he doesn't has the time. There are some bugs in it however. I asked him if i may upload it so other people can have a look at it. So a big thanks to Thugie first for creating it 
Features:
Bugs:
Installation
Download R1163+[attachment=200]

Features:
- /login
- /register
- Username changes to Guest(number) if that player isn't logged in
- Disable all block placement/digging.
- Kick everyone when the server reloads
- Kick a player when there is already someone with the same username
- MD5 encryption
Bugs:
- User char stays Guest(number)
- in the Minecraft player list (tab key) there is a Guest(number) + real char
- server saves the Guest(number) coordinates, so you can randomly spawn elsewhere when the server reloads. (the Guest(numbers) get reset if you reload)
Installation
- unzip tAuthentication.zip.
- copy the folder to the Plugins folder.
- add a Guest rank in the groups.ini with at least auth.register and auth.login as permissions.
- Add NewPlugin=tAuthentication to the settings.ini
- PROFIT!!
Download R1163+[attachment=200]