Scientific Linux is a distro based on RHEL made by Fermi Labs , CERN and others labs and universities. Its main aim is to standardise the Linux used in labs all over the market. It has all features available in RHEL as it a compile version of RHEL from its source code. Its latest release is SL 5.4 featuring:

Graphviz 2.24.0-1 a utility for charting and graphing.
qemu-0.9.1-11 for kvm
R R-2.8.1-1: graphics and computing language.
gcc44: gnu compiler
Apachehttpd-2.2.3-22
tidy : tidy-0.99.0-12.20070228.
This distro is mainly for the physics labs which require simulations of various models. Not recommended for common use. Though I think you can use as substitute of RHEL as it can install all its software. If you think NASA use it then you might be wrong as last time I checked they were using fedora and rhel.