FLENS is short for Flexible Library for Efficient Numerical Solutions. This C++ can be used as a builing block for the implementation of other (higher-level) numerical libraries or numerical applications. It is a C++ library (requires a C++11 conform compiler). Easy install, as FLENS is headers only. It gives you Matrix/vector types for dense linear algebra; a generic (i.e. templated) implementation of BLAS; and a generic reimplementation of LAPACK. If high performance BLAS libraries like ATLAS, GotoBLAS, etc. are available, you simply can link against them and boost performance.
DAC (Dynamic Agent Computations) is a novel software framework designed for implementing multi-agent systems that describe parallel computations. The whole system is easy to configure and extend, but also very efficient and scalable. Moreover, the technology that is used (JMS, Cajo, JMX) ensures high reliability of the framework, which can be used in a production environment.
LifeV is a finite element (FE) library providing implementations of state of the art mathematical and numerical methods. It serves both as a research and production library. It has already been used in medical and industrial contexts to simulate fluid structure interaction and mass transport. LifeV is the joint collaboration between four institutions: École Polytechnique Fédérale de Lausanne (CMCS) in Switzerland, Politecnico di Milano (MOX) in Italy, INRIA (REO, ESTIME) in France, and Emory University (Sc. Comp) in the U.S.A.