ATLAS (Automatically Tuned Linear Algebra Software) is an instance of AEOS (Automated Empirical Optimization of Software) from the Dept. of Computer Science of the University of Tennessee, Knoxville, presently with emphasis on the BLAS (Basic Linear Algebra Subprograms), a widely used linear algebra library (mainly operations with matrices and vectors). ATLAS is ***really*** optimized for the architecture for which it is compiled.
Thanks for the info on that one :) I have added it to the use guide, so at next website update, the entry should be in there.