I wanted to mention something about the Zimmerman's Clinical Electrocardiography, that I should have when I responded to the OP. It provides lots of ECG's, but the analyses are the key part. It systematically tells you rate, rhythm, axes. Then it goes through and reports on each feature of the ECG (hard data). Afterwards, it synthesizes that data to give an interpretation (LQTS, low limb voltages, RVH/LVH criteria, etc.). It follows-up with a discussion, sometimes with references, that gives the full implication of that interpretation. All of this on a single page per ECG, so it makes for a very quick read.
Many good ECG databases exist, like the one mellow yellow mentions. While they're great for practice, they tend to focus more on glaring pathologies and miss a systematic way of approaching ECG's. I mean, it's great if you catch that a guy's hyperkalemic or having an MI, but if you miss that he's in Mobitz Type II AV block or has features consistent with a ventricular aneurysm, then the resulting management could be disastrous for the patient.
It's just like noticing that pleural effusion on the chest x-ray, but missing the slight increased density in the lung apex, suggestive of a pancoast tumor.