Fish oil is an often overlooked cardioprotective supplement. Below is my most recent literature search on it.
Fish consumption and fish oils rich in Omega-3 fatty acids have been shown to be cardioprotective in abundant studies over the past decades. There is additional research that shows that it helps lower cholesterol, prevent atrial fibrillation, reduce mortality in congestive heart disease patients, and stabilizes atherosclerotic plaques. 
How much do I take? Studies show that people with cardiovascular disease should consume at least 1000 mg/day of oil from the oily fish, flax seeds, or fish-oil supplements. People without existing cardiovascular disease should consume 250-500 mg/day. 
What is the best source for fish oil? Fish oil supplements do not contain some of the other trace minerals such as Vitamin D, selenium, and antioxidants; therefore, some proponents recommend consuming fish instead. 
What’s the difference between quality of fish oil supplements? Fish oil supplements also come in various qualities. For example, there are heat-processed fish oils and cold-processed fish oils. On average, the cold processed fish oils will contain more of the Omega spectrum (i.e., Omega 3, 6, 9, 12, etc.) than the heat processed.
1 Roth EM, Harris WS. Fish oil for primary and secondary prevention of coronary heart disease. Curr Atheroscler Rep. 2010 Jan;12(1):66-72.
2 Lee JH, O’Keefe JH, Lavie CJ, Harris WS. Omega-3 fatty acids: cardiovascular benefits, sources and sustainability. Nat Rev Cardiol. 2009 Dec;6(12):753-8. Epub 2009 Oct 27.