What are the biggest (most serious) press release portals in the UK/USA?

[leetweb2012]

A customer is releasing a brand new and world wide unique product in the next few weeks.

I want to send out press releases to the biggest and most serious press release portals to get this news out in the world.

Can you recommend me some english portals in the UK / U.S.A. and maybe Australia? 
It's no problem if they are expensive and i don't need links in every case.

Thanks.

[conversiontactics]

We use http://uk.prweb.com/ its bit expensive but good value for money, and http://www.express-press-release.net/ which is cheaper then prweb.

[AdamThompson]

http://www.prnewswire.com/ is a good portal with a lot of bloggers and journalists that use it.

I think you will find that individual outreach is needed, unless you're already working with a well-known brand.

[leetweb2012]

Thanks, no the brand isn't known very well. It's a startup.

[rodnic66]

There's also a UK version of the site at http://www.prnewswire.co.uk/

(Full disclosure: I work for PR Newswire)