List tiles used

How can I find out which tiles have been used in a slippy map?

For instance with zoom = 2 there should be 16 tiles, how can I find out which tiles they are so that I can download one or more tiles separately or, even better, can select the tile via javascript to be used somewhere else on the page.

Regards, Steve.

http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames