On the Incan Trail, you will meet people from everywhere! If you don't want to do the trail, Aguas Calientes and Cusco have a lot of backpackers!

Backpackers - There are more in Cusco correct - but don't limit yourself to Cusco. You'll find lots of travellers in backpackers all across the country, from the more popular south east region, up to the remote northern tip. It rather depends on who you want to meet, and how hard you want to party, or how much culture you want to explore. If the backpacker has a bar, it will have a younger party crowd, if it's close to a UNESCO site, it will have more cultural crowd. If it's in a remote place, it will have more hardcore backpackers, think of this when you go and Enojoy!

Cusco, for sure. Lima is quite popular with foreigners. Try to stay at Miraflores neighborhood. In Cusco, you'll meet people from aroung the world. Special place. Good luck!

there are some great english and irish bars where all the tourists hang out

Go to Cuzco. That is it. There are more tourists there then locals. It is actually quite ridiculous.

If you stay at popular hostels like the ones listed in the Lonely Planet or Hostelbookers.com, you will have no problems meeting people!

only at the central station

you'll find them everywhere in the big citys. Especially in the hostels and cafe.

try www.travbuddy.com or www.travelpod.com and look in the forums sections for travel buddy wanted or post your own. Hostels are great once you to there.

Hl Adam, You are sure to meet lots of travelers in Peru - try popular hostals, pizza establishments, tourist attractions, public transport, churches, and night spots. Good Journeys!