A good affiliate marketer is always on the lookout for new niches with low competition and high demand. The biggest niches are weight loss and anti-aging followed by products men can use to enhance their "performance".
A niche that has been mostly neglected, but has a lot of potential and has been growing a lot of the last year is the market for "Female Libido Enhancement" products.
Thanks to attention from the mainstream media, this niche is growing faster than any other niche out there. Even major talk shows have featured the subject and reviewed some of the products. As the subject becomes less taboo, I expected the niche to grow even more. Webmaster Guide blogged a list of affiliate programs that sell these types of products. I recommend SellHealth, but it's always a good idea to diversify and send traffic to several different programs at the same time.