- CLAPACK (f2c'ed version of LAPACK)
- GSL - GNU Scientific Library
EDIT Thanks to comments from osgx:
- CLAPACK is the library which uder the hood uses very high-performance BLAS library, as do other libraries, like ATLAS.
- GSL is a standalone C library, not as fast as any based on BLAS. However its performance is still quite good (not extremely good though) and is used quite often; mostly because of its portability.
I know both LAPACK and GSL and I can recommend both of them. LAPACK is very low-level library and GSL may be more comfortable to use, but speaking of good performance -- BLAS-based library it is.
