I think a common answer would be "text files." And of course, grep and awk.

We're replacing at least one collection with text files. That's half Mongo's fault, half our fault for using it to store the kind of info we're storing.

I'd never replace awk.

Mainly in-house made no-sql databases.

I don't think MongoDB should be replacing systems, it should rather be used as a data store for things that otherwise wouldn't be stored because of the high throughput needs, but aren't necessarily critical data.

I've worked at enterprise companies with MongoDB deployments.

It is replacing Oracle databases which everybody is sick of. And given that most companies aren't clustering it is pretty much a simple drop in and replace scenario.