This is not from me, it's from the owner of BigNews.biz Is Your Free Press Release and World News Distribution Center - and it is an EXCELLENT resource. There are a lot of free press release sites out there that are just a complete waste of your time.
From my experience - the free press release sites that seem to send traffic, get to the top of the search engines, and stay at the top of the search engines, are: BigNews.biz Is Your Free Press Release and World News Distribution Center, Free Press Release Distribution, www.free-press-release-com, i-newswire.com, PressReleasePoint | Free Press Release Distribution Website | Submit Press Releases, and Press Release Distribution & Free Press Release Submit @ 1888 PressRelease.com
And www.webwire.com gives fantastic results for only $19.95
If I wanted to launch an online publicity campaign and was pressed for time and money, I would use those sites.