Several implementations of the FITS WCS standards are available:

The WCSLIB software distribution (i.e. this library) may be obtained from http://www.atnf.csiro.au/people/Mark.Calabretta/WCS/. The remainder of this manual describes its use.
wcstools, developed by Doug Mink, may be obtained from http://tdc-www.harvard.edu/software/wcstools/.
AST, developed by David Berry within the U.K. Starlink project, http://www.starlink.ac.uk/ast/ and now supported by JAC, Hawaii http://starlink.jach.hawaii.edu/starlink/.

A useful utility for experimenting with FITS WCS descriptions (similar to wcsgrid) is also provided; go to the above site and then look at the section entitled "FITS-WCS Plotting Demo".
SolarSoft, http://sohowww.nascom.nasa.gov/solarsoft/, primarily an IDL-based system for analysis of Solar physics data, contains a module written by Bill Thompson oriented towards Solar coordinate systems, including spectral, http://sohowww.nascom.nasa.gov/solarsoft/gen/idl/wcs/.
The IDL Astronomy Library, http://idlastro.gsfc.nasa.gov/, contains an independent implementation of FITS-WCS in IDL by Rick Balsano, Wayne Landsman and others. See http://idlastro.gsfc.nasa.gov/contents.html#C5.

Python wrappers to WCSLIB are provided by

The Kapteyn Package http://www.astro.rug.nl/software/kapteyn/ by Hans Terlouw and Martin Vogelaar.
pywcs, http://stsdas.stsci.edu/astrolib/pywcs/ by Michael Droettboom.

Java is supported via

CADC/CCDA Java Native Interface (JNI) bindings to WCSLIB 4.2 http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cadc/source/ by Patrick Dowler.

and Javascript by

wcsjs, https://github.com/astrojs/wcsjs, a port created by Amit Kapadia using Emscripten, an LLVM to Javascript compiler. wcsjs provides a code base for running WCSLIB on web browsers.

Recommended WCS-aware FITS image viewers:

Bill Joye's DS9, http://hea-www.harvard.edu/RD/ds9/, and
Fv by Pan Chai, http://heasarc.gsfc.nasa.gov/ftools/fv/.

both handle 2-D images.

Currently (2013/01/29) I know of no image viewers that handle 1-D spectra properly nor multi-dimensional data, not even multi-dimensional data with only two non-degenerate image axes (please inform me if you know otherwise).

Pre-built WCSLIB packages are available, generally a little behind the main release (this list will probably be stale by the time you read it, best do a web search):

archlinux (tgz), https://www.archlinux.org/packages/extra/i686/wcslib.
Debian (deb), http://packages.debian.org/search?keywords=wcslib.
Fedora (RPM), https://admin.fedoraproject.org/pkgdb/package/wcslib.
Fresh Ports (RPM), http://www.freshports.org/astro/wcslib.
Gentoo, http://packages.gentoo.org/package/sci-astronomy/wcslib.
Homebrew (MacOSX), https://github.com/Homebrew/homebrew-science.
RPM (general) http://rpmfind.net/linux/rpm2html/search.php?query=wcslib, http://www.rpmseek.com/rpm-pl/wcslib.html.
Ubuntu (deb), https://launchpad.net/ubuntu/+source/wcslib.

Bill Pence's general FITS IO library, CFITSIO is available from http://heasarc.gsfc.nasa.gov/fitsio/. It is used optionally by some of the high-level WCSLIB test programs and is required by two of the utility programs.

PGPLOT, Tim Pearson's Fortran plotting package on which PGSBOX is based, also used by some of the WCSLIB self-test suite and a utility program, is available from http://astro.caltech.edu/~tjp/pgplot/.