Botstalker is a simple web app which lets you monitor your bots direct message channels with other users. Enter your bot's API key and select a user. This allows you to easily debug messages your bot might be sending other users. I build this tool after discovering a slack bot I create had been messaging a lot more than expected - it turns out he was chatting away with Slackbot!
Botstalker is released under the MIT licence and can be found here on Github.