Links

General information about HMAC as described on Wikipedia, the free encyclopedia.
http://en.wikipedia.org/wiki/HMAC

General information about SHA (including SHA-256) as described on Wikipedia.
http://en.wikipedia.org/wiki/SHA

More information on HMAC-SHA256-128 can be found in the following documents:
FIPS-180-3 (Describes SHA (including SHA-256))
http://csrc.nist.gov/publications/drafts/fips_180-3/draft_fips-180-3_June-08-2007.pdf

FIPS-198-1 (Describes HMAC)
http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-1_final.pdf

RFC4868 (Describes HMAC-SHA256-128 and how the hash value is truncated)
http://www.isi.edu/in-notes/rfc4868.txt

Reference implementation of HMAC-SHA-256 open source code
http://beecrypt.sourceforge.net/doxygen/c/index.html

Additional example of reference implementation of HMAC
http://www.lysator.liu.se/~nisse/nettle/