Open Source
MySQL:
!Limited SQL, not good for OLAP (but getting better all the time)
!Limited transaction support but fast lookups
!Extremely widely used, e.g., web servers (Linux Apache MySQL PHP - LAMP)
PostgreSQL:
- Rich SQL features, decent query optimizer!Not as widely used but popular in academia
SQLite:
- SQL transactional DB, server-less, self-contained, zero-configuration
Berkeley DB!No SQL!!
Embedded database, a few hundred KB size
Basically a library of disk-based data structures with support for transactions
MapReduce, BigT able, PNuts, Dynamo, and others
No SQL
Used for data and text analysis (search engines etc) or to keep live data
Many other specialized systems for warehousing and continuous queries