SEO - Ghost domains

Duplicate sites can help to raise your website profile.
Search engines rate the domain name very highly when it comes to matching a query, thus if you buy additonal domains which contain your keywords, you may harvest more traffic.
If you have, for example a website for mobile phones, let's say it's called 'mobilephones.com' you could also mirror your website on 'cellularphones.org' or 'cellphones.net' etc.. etc.. This would help to pick up hits from searches which contain the terms 'cellular phones' or 'cellphones'. You could just use a redirect on the homepages of these domains, or configure all the links contained in the site to jump back to your main site.
Although this may seem an unwieldy promotion tool, bearing in mind the reasonably low cost of domain hosting nowadays, it is probably fairly cost effective. I would recommend duplication of the entire site and linking every page back to your main domain, this way you get a combination of the new domain name and your existing content to boost your ranking - it would make sense to have a link to your new domains on your link page just to complete the circle for reciprocal linking. Make sure you submit your new sites in the same way that you submitted your main domain for maximum impact.