If you guys don't have Portal Playground yet, get it here: {LINK REMOVED}http://tinyurl.com/353xdp

Drop the BSP in your maps folder (Steam/steamapps/<yourname>/portal/portal/maps) and load it in game by opening the console (enable it in keyboard advanced settings) and type 'map portal_playground_v1'. If it looks way too bright, type 'mat_fullbright 1' into the console.

Protip: It's easy to get the Long Jump achievement in this map, but achievements don't count if you have cheats on.