Online service for crowd sourcing for keyword suggestions?

I am looking for an online service where it utilizes crowd sourcing or humans for keyword suggestions. Meaning if I want to create a CPC ad for some product or service, I want to get ideas from humans (not automated tools) of what get keywords or phrases they would use when they do a search online for that product or service.

Any sites providing such a service? Would Amazon's Mechanical Turk be sufficient?

You can check Trada. It crowd sources SEM experts to build and manage paid search campaign across search engines.

oDesk, Elance or Mechanical Turk are all excellent services that can do this for you - I find that Mechanical Turk is cheapest, but has the lowest quality results on the whole. Elance is more expensive, but tends to have higher quality results. oDesk is in between.

I tend to use oDesk, purely because there is no coding required (you can get people to log into your CMS, rather than having to use Amazon's API's to interface with MechTurk), and the results are reasonable. You can further improve your results by guiding and managing your workforce, firing people who perform badly, and encouraging those who deliver high-quality work.