OrbitSoftware
TLE (two line element) computation
Visualization
SaVi satellite constellation visualization
Online apps
NASA 3D Realtime Satellite Tracker (Java)
PREDICT Satellite tracker - Debian & Ubuntu packages exist
Other online software
NORAD SGP4/SDP4 Implementations - C++/C# ports of the NORAD algorithms for Windows
Astro-SpaceTrack-0.039 - Perl module at CPAN for TLE retrieval