I’ve found 24-7 Press Releases (http://www.24-7pressrelease.com/) to be pretty effective, and I’ve also heard good things about PR Newswire (http://www.prnewswire.com/).
CAP previously used PR Web and Send2Press a lot, but both those sites now forbid any content related to online gambling. (So does PR LEAP, but I’ve never had a very high opinion of them anyway.)
There’s also MarketWire and BusinessWire … if you have $1,000+ to spend on each press release! 