Compass

A compass is an item that points the direction towards the Hub.

[edit] Possible results

"The needle is pointing roughly towards the <direction>, or somewhere around <number> degrees." - The needle is pointing towards the Hub.
"The needle is rotating freely, unsure of which way is hubwards." - The room you're in isn't connected properly to the rest of the Disc. This can be a permanent situation (as with passage rooms and a few other places) or a temporary one (in which case moving a few rooms and coming back should fix it).
"The needle is rotating rather quickly, causing the box to vibrate and hum." - This seems to occur at the Hub itself, and in some nearby areas of the Ramtops.
"The needle is rotating very rapidly, causing the box to shake violently." - This seems to occur at the Hub itself.