It's very 'Unix-y' in the sense that it's supposed to do this and only this.
I created it because I wanted to have a way to make notes without being dependent on apps. With moncat, I can use any e-mail client to incrementally create larger text files.
Currently, I'm using it to write a journal in Markdown that is automatically converted to HTML. How that works: I e-mail journal entries to myself, put them in a mailbox folder, and periodically compile the journal using a cronjob.
moncat accepts some basic commands that you can put in the subject line of the e-mail. For example, you can reorder items to be concatenated. It also handles attachments and nested folders.
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Yeah.. so there is no documentation and the code is pretty shit, since I'm the only one using it. The upside is that the code is also pretty small (around 350 LOC Python in total).
So, just in case anyone is looking for a tool like this... here you go! ;)