In MongoDB docs here, you will find that the listed MongoDB storage engines are:
- In-Memory Storage Engine
Are there any other storage engines available? Must be free, or even open source.
Available storage engines in the MongoDB Community Server (as at MongoDB 3.6) are:
MongoDB Enterprise Server (commercial edition) adds:
Outside of these officially supported storage engines there is also MongoRocks (RocksDB) which you can either build from source or find in a community distribution like Percona Server for MongoDB. Development activity on MongoRocks (as compared to development activity on WiredTiger) is significantly less active in terms of contributors and commits.
I'm not aware of any third party storage engines with significant adoption. My recommendation would be to use the default storage engine (WiredTiger) if you want the most thoroughly tested and supported option with timely access to new MongoDB features.