Ours were originally on Gerty Guinea Pig muesli, until I found out how bad it was. Then switched to pellets (Pets at Home), but for some reason gave Choccy UTIs, so switched again to Excel (the green bag, I forget the name). They're currently eating the Excel Blackcurrant and Oregano pellets with a small handful of Science Selective mixed in (as I got this free recently). The Science Selective seems to be preferred over the Excel (as there's always none of this left but Excel is left behind), so I will probably buy some more of this when the packet runs out!
And of course they get the usual hay, cucumbers, carrot and other veggies (depending on what's available at home!)
It's very interesting to see what food/pellets others are fed! I always wondered if people rotated foods, as it must get boring eating the same stuff everyday for the piggies... Maybe?