Remember the clones in the gunships in the Geonosis arena? Remember the clones that stormed the Jedi temple? (bear with me through the prequel references), the clones that killed all of the jedi in the second half of ROTS? The Stormtroopers that boarded Leia's "diplomatic" frigate?
All the handiwork of Vader's fist. The legendary 501st legion.
Specially trained during the Clone Wars to be the Navy SEALs of space, they totally raped a buttload of droids. After they won the war with the Seperatists (I like calling them the Confederates Without Slaves), they were deployed by the Emperor as his personal Jedi hunting battalion. They were deployed to Kashyyyk, Ord Mantell, Corellia, Bothuwai and a number of other Jedi-friendly worlds to eradicate to infernal Jedi scum. After he believed all the Jedi were dead, he used them as a strike team to take out key Rebellion outposts and bases.
After the Rebellion revolution and the re-descent into bureaucracy, and the "New Republic"'s treaty with the majestic Galactic Empire, the Imperial "remnant" to most New Republic sissies, the New Republic became the GA (the Galactic Alliance), with the GE and the NR under one banner, and it lasted in relative peace for about 100 years, up to 130 ABY, when the new Sith Order of One took control of the galaxy, the GE was forced into a guerrilla war against the Sith. The Empire AGAINST the Sith. Imagine that. Aaaanyways, Emperor Roan Fel resurrected the 501st from the tales and testimony from the old glory days of the 501st, and used it to strike at the Sith from their heavily fortified planet-base, Bastion.
The story of the 501st is still an open and blank book, with plently of room for expansion. In any era really. Clone Wars, Galactic Civil War, the Yuuzahn Vong War, the One War. I really hope that the authors at Lucasarts (WITH NO HELP, ADVICE, CONTROL OR INFLUENCE FROM GEORGE LUCAS) expand this amazing facet of the Star Wars galaxy.