MuPuPriNT
A primality/compositeness tester for different types of numbers.
...It is basic, yet full-featured, and offers several different testing methods for each type of number.
MuPuPriNT is built on the GNU Multiple-Precision (GMP) integer + rational library, and is available for Windows, OS X, and Linux. Source code is available as well, and can be built on any machine with Qt installed.
As of version 2.0, MuPuPriNT is built on the cross-platform Qt application framework. There are currently two versions: MuPuPriNT and MuPuPriNT Extended.