I think that L-arginine might be the most disputed conception aid. Some sources state it improves quantity of eggs but decreases quality.
L-arginine converts in the body to nitric oxide. Nitric oxide causes vasodilation, and that may help to increase blood flow to the ovaries. Since we can't see what's happening at that molecular level, we don't know for sure, but doppler studies have shown decreased resistance to blood flow with L-arginine. That may increase the flow of FSH and other hormones to the ovaries. This idea was supported by a study from 1999 (Battaglia C, et al. Human Reproduction, vol.14, no.7, pp.1690–1697). In that study of 34 patients randomized to receive L-arginine or not during IVF, three previously poor responder women in the L-arginine group got pregnant, and none got pregnant in the other group.
Then Battaglia published another study in 2002 that showed opposite results in normal responder patients. The authors found that the women who did not receive L-arginine supplmentation had the same L-arginine follicular levels as the women who were supplemented in the first study. Remember that the first study was in poor responders, so the authors wrote that there is clearly some L-arginine that is needed, but maybe not too much. The egg quantity of both groups of women in this study was about the same, but the embryo quality was much better in the unsupplemented group. (Human Reproduction, Volume 17, Issue 3, p. 659-665)
Another study in 2010 by another group seemed to question the value of L-arginine. It showed that increased levels of L-arginine measured in follicular fluid led to a decrease in the number of eggs and ultimately embryos conceived. The problem with this study is that the patients weren't supplemented with L-arginine, so it doesn't correlate to say that supplementation is a bad thing due to these results.
All in all, it seems that L-arginine may be most useful for poor responders to previous IVF cycles. For those who are responding normally but just not getting pregnant, it doesn't seem too helpful. For those who are trying to conceive naturally, that's up in the air.