Carmine is a bot for the Jabber instant messaging protocol. The bot itself is just a very lightweight console application that takes care only of the Jabber communication itself, but it can easily be extended using plugins and the MEF. The download (see below) is bundled with a useful "Reminder-Plugin".
This is some communication with Carmine in Google Talk, showing some functionality of the "Reminder-Plugin":
You may download Carmine here. The archive contains the source code as well as a compiled version.
Sure, since I'm using Carmine on a daily basis, it is running and available at the Jabber network. Just add "email@example.com" to your Jabber contacts.
You can do whatever you want with the source (WTFPL), but note that some of the third-party components used by Carmine are licensed under the GPL or other licenses.
The author of Carmine is me, Hannes Sachsenhofer. I wrote it as an exercise for a course covering C# at the JKU. You may contact me under hannes AT sachsenhofer DOT com.