DB2 Express-C is the free edition of the DB2 for Linux / Unix  / Windows database. Unlike Microsoft and Oracle express offerings, DB2 Express-C does have a huge limit on the size of database! The latest edition (10.5) relaxes its memory restriction by 400%! It will now use up to 16GB of memory. Furthermore DB2 Express-C can be used in production and it can be redistributed with your applications.

Here is how the three major express from IBM, Microsoft and Oracle and their limitations:

DB2 Express-C is available for Windows (x86 / x64), Linux (x86, x64, Power Systems), Solaris, and Mac OS X. DB2 can be easily managed from the command line or with the free tool, IBM Data Studio.

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Please note that above was true on September 27, 2013.
Always verify the vendor’s website for their latest offerings or change in limitations.
Previously this post stated that there was no database size limit in Express-C. The announcement letter for 10.5 mentions a limit. This limit may have been there in earlier release but was never noticed before.