I found the following on Wikipedia:
The original trilogy:
1. The Awakening (1991)
2. The Struggle (1991)
3. The Fury (1991)
The fourth volume in the original series:
4. Dark Reunion (1992)
The new trilogy, The Vampire Diaries: The Return:
5. Nightfall (February 10, 2009)
6. Shadow Souls (due March 16, 2010)
7. Midnight (not yet released)