Redis is a key-value database. It is similar to memcached but the dataset is not volatile, and values can be strings, exactly like in memcached, but also lists and sets with atomic operations to push/pop elements.
Redis is pretty fast!, 110000 SETs/second, 81000 GETs/second in an entry level Linux box.
MongoDB is a high-performance, open source, schema-free document-oriented data store that's easy to deploy, manage and use. It's network accessible, written in C++ and offers the following features:
* Collection oriented storage — easy storage of object-style data
* Full index support, including on inner objects
* Query profiling
* Replication and fail-over support
* Efficient storage of binary data including large objects (e.g. videos)
* Auto-sharding for cloud-level scalability (Q209)