ChatterBlocker is an app designed to help you concentrate and be more productive in noisy environments. It doesn't somehow cancel or block out noise, but rather it's intended to "blur unwanted speech," rendering conversations unintelligible so they won't distract you.
ChatterBlocker plays a combination of voices, nature sounds, and music, in several preset options or using an advanced interface that lets you mix and match. Since the effectiveness of an application like this is so peculiar and personal, this ability to tweak is extremely useful, no matter what particular sound fetish works for you. (Need the high-pitched murmuring of women accompanied by goats wearing bells to think straight? No problem!) ChatterBlocker seems to work much better over headphones (which they caution against using too much), and the voices by themselves--what ChatterBlocker calls "walla" or "cafeteria noise"--tend to do the best job at masking speech.
ChatterBlocker takes some getting used to and it's spendy for its functionality, but if you work in a conversation-filled environment and find yourself frequently distracted, the 60-minute demo of ChatterBlocker is absolutely worth trying.