There really isn't much to take that will signifcantly help endurance. Creatine helps some in bicycling. I've loaded up on 30-40g of creatine before hard races and I feel I have some increased muscular endurance. This seems to help in short intense efforts like a short steep hill etc. However my legs felt "pumped" all day, the water retention I suppose. I suppose creatine loading could help in running, but cycling is more of a power sport so the benefit is noticeable. I take iron supps after long hard workouts to help with red blood cell production. Don't take iron every day though, that will lead to similar symptoms as an iron deficiency. Some steriods will increase erythropoiesis-RBC production. I've heard some OTC prohormones will help some as well, but if they do the effect is much lower than w/steriods like anavar and EQ. Supposedly ectosteroids will help erythropoiesis, but again this is debatable.
Honestly the best thing is to just train more and harder and rest lots as well!! Long hard endurance sessions stimulate the body to create more red blood cells. Not getting enough rest (and poor nutrition) kills your body's ability to recover which in turn slows or hinders erythropoiesis which lowers your hematocrit level (% of RBCs in blood). Top endurance athletes have a hematocrit close to or above 50%. Most people have unrealistic expectations about how to improve dramatically w/o putting in the training. For me to remain competetive at an elite level as a cyclist, I have to train 20-30 hours a week. All my competitors do!! So I don't do much besides work and train, but thats the life of an elite athlete.
EPO probably has the greatest effect, but it is so expensive and honestly you could probably naturally attain the same level thru just training more; it will just take much more time. EPO works great, but it has bad side effects if used improperly. You should only take EPO if you are one the best in your sport and want that extra (albeit illegal) edge to win a major championship, Olympic, or World medal.
Endurance supps that have a significant impact are hard to find. Endurance is all about just putting the time in.
My .02
FHG