Great contributions!
(Concentrated) Fish Oil extract has been mentioned. Also reduces very low-density lipoproteins (VLDL) and small low-density lipoproteins (LDL). Niacin too. Beta-Glucan (either as supplement or straight from high-fiber foods), and raw nuts (almonds and walnuts), too. They lower triglyceride levels indirectly via lowering the "glycemic index" of foods.
I once read that when the level of triglycerides rises higher than 60, this triggers the build up of several abnormal hidden particles in the blood. These include very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), and low-density lipoproteins (LDL). Beneficial high-density lipoprotein (HDL) levels decrease with a rise in triglyceride levels.
For raising HDL levels, niacin supplementation, weight loss, calcium pyruvate supplementation (for weight management), fish oil, policosanol, chromium, and eliminating hydrogenated fats (cut HDL and raise LDL) and refined starches from one's diet are useful strategies.