Yohimbe, which is still sold legally in the US (since banned in Canada) is a well-known aphrodisiac. Ginseng has also been used for this purpose (among other things) for centuries, but personally, it's never done anything for me.
Personally, i'd recommend regular exercise and keep your weight down. Eat well. And most importantly, keep your blood pressure to normal levels by not ingesting agents that would cause it to rise. For example, i used to take ephedrine and realized over time that it caused me to have hypertension, which led to problems maintaining an erection.
And finally, nothing that you can buy over the counter will be as effective as either viagra, cialis or levitra.