Europee

Arrhythmology:
Belgian Working Group on Cardiac pacing and Electrophysiology
ESC Working Group on Arrythmias
Pacing and Clinical Electrophysiology
The Danish Pacemaker Register

CHF & Related:
Cardioreparation Web Index
Heart Attack Survival Calculator
Hypertrophic Cardiomyopathy Association

Circulatory:
Virtual Congress of Heart Rate Variability

Computers:
Computers in Cardiology
ESC Working Group on Computers in Cardiology
SUMMIT - Computer Education

Non-Europee

Echo & Non-Invasive:
Echo-web (resource for echocardiographers)
ESC Working Group on Nuclear Cardiology
ESC Working Group on Magnetic Resonance
Foetal Electrocardiography
Freeman Echocardiology
Esonography.com

Emergency:
Online Emergency Services Resource

Invasive & Surgery:
Heart Surgery Forum
How to Perform a Safe & Effective TASH
Invasive Interventionelle Kardiologie (German) 

Pharmacology:
Mc PharmNet: Molecular Pharmacology at McMaster

Transplant:
TransWeb-Transplantation & Donation