How can I get 20,000 unique visitors to my web site, on a very small budget?

[Jakester]

Of course press releases are quite valuable for getting mention in news outlets, I'm sure you have tht handled, let me just mention prweb.con is the premier distributor, charging some $80.

I imagine to be cost effective, you would want state specific geo targeted ads, which somewhat narrows the options. I understand perfectly optimized adwords campaigns for non affiliates with keyword quality ratings of 10 might the cost down near 10 cents a click. Similar ppc on yahoo and ms/bing might add another 35% ppc visitors.

I could image paid banners on employment or business sites might help a bit.

You could experiment with CPM media buy banner ads if you can find any that can geo target down to your level, they are less costly than mainstream cpc ads, some of the best have a $100 or $200 minimum.
Advertise.com decent quality, $100 min
AdBrite.com cheap lower quality traffic, good one for affiliates, cpc is safer, $100 min
Clicksor.com many delivery channels, $100 min
MediaTraffic.com CPV, target competitors $200 min