A couple of recommendations. On October 6th Lily (my employer) and I went to a performance by Methera, and it was outstanding. From the official website:
Methera unites the rich texture of the string quartet with the depth and integrity of traditional music from England and beyond. Four fine young musicians with individual traditional styles weave old and contemporary tunes into the tapestry of the string quartet, exploring the power and diversity of sound it has to offerThe crossover between folk and chamber music isn't new. The description reminded me of a detail I heard a long time back about Turlough O'Carolan, the great Irish blind harper, being influenced by Corelli; see these Albuquerque Baroque Players programme notes for interesting discussion of such fusion going on historically. But Methera brings a very fresh and very English spin on the concept.
Methera's lineup is standard - two violins, viola and cello - but, unusually, Methera play facing inward to tightly coordinate their playing. You can hear some of their pieces at www.myspace.com/methera, which give an example of the flavour of their music - even if the online samples don't do justice to the dynamic range and the marvellous atmosphere, intensity and intimacy of the live performance. If you get the chance to go to one - it'll have to be their 2010 tour now - both Lily and I highly recommend them.
And then there's the Finnish rock band Apocalyptica (see Wikipedia and their official site) whose core lineup is four classically trained cellists. They produce a remarkable fusion of metal and string quartet styles, much of their work comprising covers of songs by heavy metal bands such as Metallica (for instance, "One", embedded above, is Metallica's "...And Justice for All"; and another of my current favourites by them, "Fade to Black", comes from the Metallica song of the same name). Their collaborations are excellent too: check out "Helden", a German cover of David Bowie's "Heroes" with vocals by Rammstein's Till Lindemann, and their performance of Rammstein's "Seemann" with the scary Nina Hagen.
Addendum: November 9th 2009. "Helden", the German cover of "Heroes", is especially pertinent today, which marks the 20th anniversary of the fall of the Berlin Wall. The lyrics, Bowie said, were inspired by seeing a couple defying the system by making love on top of the Wall. Rammstein themselves, by the way, have East German / Berlin roots, as does Nina Hagen (I'm wondering if the strongly trilled R is a regionalism).