-- Edward Gibbon; Historian
1. Texas Secession Movement (links to secessionist organizations. Note: Some are radical; others are clearly home-made sites with a distinct lack of critical thought, as well as very bad grammar and spelling).
2. Republic of Cascadia (Cascadian Independence Project. Part joke; part sovereignty movement, the Northwest has entertained the concept of secession since the 1930's [see 'State of Jefferson' link, below])
3. Middlebury Institute (a think-tank of secessionist and sovereignty researchers, headquartered in New York)
4. Tenth Amendment Center (discussion and clearinghouse of various Tenth Amendment/Secession/Sovereignty sites, articles, and other info. [Note: Many of these are serious conspiracy-theorists. It's up to you to sort out the tinfoil-hatters from the more-objective]).
5. Free State Project (New Hampshire sovereignty)
6. Hawai'ian Sovereignty Movement (This site advocates Hawai'ian secession, not a Tenth Amendment argument).
7. State of Jefferson (The granddaddy of sovereignty/secession movements in the 20th and 21st century, this was originally a proposal to create a 51st state - now; there are many who believe it should be a separate country. Some interesting secession/Tenth Amendment arguments here, too).