For commercial packages that support multiple element types, look at M++ from Dyad [phone in the USA (800)366-1573, (206)637-9427, fax (206)637-9428], or the Rogue Wave matrix package [(800)487-3217, (503)754-3010, fax (503)757-6650].
For details of other matrix packages look at the listing from Keith Briggs. A recent version of this is in file kbriggs.txt included with this package. Also look at Ajay Shah's list of free numerical software in C and C++. The file is numcomp-free-c.gz in pub/C-numanal on usc.edu. Nikki Locke produces a list of C++ libraries in general - see pub/usenet-by-group/comp.lang.c++/c++_FAQ/libraries in rtfm.mit.edu.
Also look on Compuserve in the forums of the various C++ compilers.