You can get keys from NPC's, Slots and "Over and Under", you'll need them for a gold ring and the key wheel. The gold ring is for entering Minus21 and from the keywheel you can obtain many items like gems, bloodstones and other stuff.
KEY MAN
25 keys for 2,500 gold
50 Keys for 5 Karma Drops
75 Keys for 5 Blood Stones
100 Keys for 1 Silver
200 keys for 1 Master Crystal
300 Keys for 1 Master Diamond
400 Keys for 1 Master Emerald
500 Keys for 1 Master Ruby
3,000 Keys for 1 Gold Ring