There are several, one following the ebay model and one following the craigslist model. This thread, however, is
a) in a central location with an audience who probably checks neither of the above,
b) works for items/services which don't fit well into the other site's categories,
c) doesn't require pricing information, which is a hurdle for me at least
d) lets you see what other people WANT, instead of what they're offering.
Feel free to link to your auctions on other sites, though. What I'm trying to encourage is a detailing of what people want to see for sale or barter without commitment to actual prices, so that people can find and explore new economic niches.
I'm working on a website to do just this. Basically you post notices about what you want of have and any other information you feel like including; essentially a notice board for bitcoin traders.
It's encouraging to see interest in my project (albeit indirect). Hopefully I'll have it ready by next week.