By Metro
Metro Line 2 now runs from Cairo into Giza, although it doesn't go all the way to the Pyramids. Get off at Giza station not the terminus!. The pyramids are 8 km, 15-20 min trip due southwest on the long Al-Haram avenue that the train crosses over just before stopping at the station. That's effectively a 'right turn' from the line you've just been taking. Any number of minibuses and buses go to the site known as 'al-haram' in Arabic'; they include the green public 900 and 997 buses.
By bus
From central Cairo, the optimum way to get to the Giza Pyramids using public mass transit options is by bus routes 355 or 357 - a large white, air-conditioned coach with CTA Cairo Transport Authority on the side. Travelling every 20 minutes from the airport and Heliopolis, the bus stops or doesn't - you may have to flag it down! at the Abdel Menem Riyad Station in Midan Tahrir, next to the Egyptian Museum, before continuing out to Giza and the Pyramids. Tickets costs LE 2 - a bargain!
Even cheaper and more interesting is taking the ordinary buses 900 or 997, costing 50 piastres, from the big central bus station under the overpasses, close to the museum. There are three lanes, and they leave from one closest to the rundown controllers' booth as of late July 2010. Be careful when about getting down, most people will be honest and help you, but you may encounter scammers who take you to their camels instead of to the pyramids. For 997, the correct spot is along a long avenue, after you're spotted the Pyramids and the bus has done a U-turn and then turned left — get off when you a see a blue sign for the Light and Sound show.