Redis is a general purpose k/v data store that you run in memory. It has support for transactions and atomic operations, and it can be persisted and resumed. You can use it for caching, counting, event sourcing, lat/lng data, pub/sub, and more.
My coworker once called redis "NoSQLite", which I think is a very apt description.
Too much to not mention: davidw, antirez, and Richard Hipp (SQLite) are all current or former heavy hitters in the Tcl world. David contributed to the second ed. of Tcl and the Tk Toolkit[0], co-authored Rivet[1] (an Apache httpd Tcl module), among other things. antirez wrote Tcl the Misunderstood[2] and Jim[3], a lighter weight Tcl implementation, and Richard (drh) is a former Tcl Core Team member[4] who describes SQLite as “a Tcl extension that escaped into the wild.”[5]
My coworker once called redis "NoSQLite", which I think is a very apt description.