Nonoxynol-9

Nonoxynol-9 (N-9) is a detergent-like chemical that has been used for over 30 years in vaginal products to prevent pregnancy. In the United States, N-9 is available in a variety of concentrations and formulations, including gels, creams, foams, suppositories, and film. In addition, N-9 is used in some vaginal and rectal lubricants and in the lubricant used in some prelubricated male latex condoms.
-from the CDC

The most popular spermicide in the world, it's also recently been implicated in causing vaginal lesions, increasing the risk of STD transmission, which seems to eliminate it from the running as an HIV preventative.

Nonoxynol-9 is a horrible substance that's added to lubricated condoms and a lot of sexual lubricants. It is not as effective against HIV as was previously thought possible, and many people are allergic to it (a personal hypothesis is that nonoxynol-9 sensitivity exacerbates and makes more likely a latex allergy).

For many of my friends, nonoxynol-9 hurts when they try to use it. These are also the friends who hurt when they try to use latex. And for myself, I can deal (albeit a bit uncomfortably) with latex condoms, but add in nonoxynol-9 and I can't deal with the burning.