You only need a mobile phone with a browser. Any modern phone will work: Android, iPhone, bada or Windows Phone.
You can create your own adventures using the Lua programming language. For free.
-- hello from geolua
geo.event.join(function(event)
geo.ui.append(event.player_id, geo.widget.text[[
= Hello from Geolua
{{http://geolua.com/s/img/gamer.png}}
]])
end)
That is just a very small example. See the documentation and create your first adventure.