I am very interested in several databases right now:
VoltDB - In-memory, "fast" data, open source (+commercial backing like Riak), ACID-compliant, SQL. Created by Dr. Michael Stonebraker - who was behind Ingres, PostgreSQL, CTO @ Informix, Vertica, and now VoltDB. The pedigree of VoltDB is 2nd to none and I believe it will become very big in the coming years. There was an excellent Podcast with John Hugg from VoltDB on Software Engineering Daily. Worth a listen.
InfluxDB/InfluxData - DB focused on time-series data like IoT or monitoring. A whole stack of open source tools for ingestion, storing and analyzing/visualizing the data.
CockroachDB - love the name Like a cockroach surviving a nuclear fallout, CockRoachDB focuses on consistency in the face of failing distributed systems... even a data center outage. It's SQL-based (though it behaves more like a KV store as joins are not supported). It's open source and there has been a commercial company started to support it. I love the approach and focus on rock solid consistency/reliability.