I use PR Web as my main one. There service is good. They offer different pricing (contribution) levels. If it is a major press release or one I want to use as part of an SEO campaign then I go with the $200 contribution so I can include hyperlinks and images. Lower level press releases I usually use the $30 contribution.

Others I have researched:
PR Web – www.prweb.com
PR Newswire – www.prnewswire.com
PR Leap – www.prleap.com
EMediaWire – www.emediawire.com (Part of PR Web)
BusinessWire – www.businesswire.com
MarketWire – www.marketwire.com
EWorldWire – www.eworldwire.com