OK. Without resorting to Wookieepedia, here's my understanding of the huge number of Sith organizations in the EU.
- The Sith Race
- The Sith Empire (The Revan made)
- The Sith Triumvirate (KOTOR 2)
- The Other Older Sith Empire (TOR MMORPG)
- The Brotherhood of Sithness (that Darth Bane killed?)
- Darth Bane's Pair o' Siths, leading to Darth Vader
- Lost Tribe of the Sith
- Jacen Solo aka Darth Caedus... a Sith of one.
- The Krath, who weren't Sith.
- Darth Krayt and the Tattooed Sith of the Future
Am I missing any?