HailDB is a relational database in shared library form that can be embedded into applications. It is not a SQL database, although you can use this library as the storage backend for a SQL database. It features a clean and simple API and is a continuation of the Embedded InnoDB project. With the addition of the HailDB plugin to Drizzle, you get a full SQL interface.
HDB is a small, flexible, and efficient hybrid database, a mix between a filesystem and a database. It comes with a wide range of command line utilities for manipulating and extracting data. It's designed for both embedded and bigger systems. It currently has interfaces to C, C++, Java, and Perl.
HeliDB is a database for storing key-value pairs from a Java program. The client program creates one database object per database file. The database objects can be configured to support different capabilities. In its simplest configuration, a database is a file-backed map. More advanced configurations can give it transaction support or fast record searches by indexing records in a B+ tree.
HotCache is a transactional distributed caching system that runs on top of most RDBMSes. It provides an object-relational mapping tool that makes the use of embedded SQL unnecessary and saves tedious mapping programming. It enhances RDBMS commit functionality by adding commit-broadcast transactions where data is committed to the database and at the same time pushed to all other client caches, in essence keeping the data at remote clients hot. This improves performance by orders of magnitude (as most data access can be done locally) and reduces the load on the database server. The commit-broadcast transactions are implemented using UDP multicast for maximum network utilization. HotCache provides a sophisticated event framework that allows the client to register interest in business objects and be notified when changes are committed. The event framework is built so that it can be easily integrated with other systems' event-loops, eliminating multi-threading problems, etc. Notification can also be configured to take place in an asynchronous/multi-threaded method.
HyperSQL is like the doxygen program with Javadoc added, but for SQL. It hypermaps SQL views, packages, procedures, and functions to HTML source code listings, and it shows all the code locations where these are used. You can use JavaDoc-style comments for documentation and even embed UnitTests with them.
Infobright Community Edition (ICE) is an analytic database built on MySQL, designed to deliver very fast query response against large data volumes. It is a self-tuning, columnar database that eliminates the need to create or maintain indexes, partition data, or perform other administrative tasks.