Press releases are usually done to gain exposure and also to gain ability in SERPs.So doing press release is an effective promotion technique to improve visitors to your websites .So please make a note of it http://www.bizeurope.com/ http://www.clickpress.com/ http://www.betanews.com http://www.prleap.com/ http://www.directionsmag.com http://www.npr.org/ http://www.free-press-release.com/ http://www.downloadjunction.com http://www.openpr.com/ http://www.filecluster.com/
caxid, strangely enough, you did not mention the best ones: - prweb.com - prnewswire.com - marketingwire.com - businesswire.com
There are only a few Press Release sites that are FREE. I didn't come across any "the best free one!".