Mario & Luigi: Partners in Time

Infinite Coins in Thwomp Volcano
Inside Thwomp Volcano, you will find two tall rooms where you use the Baby Spin move to fly downwards while gathering coins. Collect all the coins that you can get, then switch to adult-Mario and -Luigi to return to the previous room. Use the warp-pipe to bring back bay-Mario and -Luigi. Use the Baby Spin move again in the next room, as all the coins will still be there. You can keep doing this to collect infinite coins..
(Thanks to Rex)