Although they have been in existence for over a hundred years, seven string guitars have become more popular over the last decade or so. The seventh string is placed at the low end of the guitar preceding the low E and is usually tuned to B so the standard seven string tuning becomes BEADGBe.
One of the first seven string guitars was used by Steve Vai with the Ibanez Jem Universe and has since been adopted by bands such as Korn who exploited the massive low end offered by the seventh string.
Today a lot of mainstream manufacturers have jumped on the seven string bandwagon and now offer these electric guitars as part of their range.