Novice
Basic ECG Tutorial - Simple ECG tutorial covers the very basic basics with crude example strips
Basic ECG Case Studies - Not too hard, but be careful!
Merck Manual - Hundreds of lab tests and their normal values
National Teaching Institute - AACN NTI 2004 Information

Advanced
12-Lead ECGs - Explained in various formats
12-Lead ECG Quizzes - Scenario based 12-lead interpretation
PA Catheters - Hemodynamic tutorials and quizzes
Clinical case studies - Radiology, ECG, and photographic scenarios

Rhythm Animation - Excellent animation showing details of what happens in several rhythms

Expert
ECG Quizzes - Comprehensive ECG Quizzes
Pacemakers galore NASPE - Interactive online courses (Register at the bottom of the page)