Video: If you've lost the key to one of your old padlocks, and you don't have any way to cut it, you can get it open with just about any weighted object.
In this video, youtuber bosnianbill demonstrates the "rapping" technique for opening up padlocks that use a claw to hold the lock closed. Normally, you'd have to use the key (or lock pick set) to turn the chamber and allow the claw to be released, but a hammer might do the trick just as well. By applying tension to the lock's u-shaped bar (as if you were trying to pull it open) and rapping the the claw side of the lock with something heavy (hammer, screwdriver, rock), the claw can shake loose and release.
While this video is specific to the MasterLock brand of padlocks, YouTube user rallock67 suggests this can be done with almost any padlock with a similar claw design. Be careful when using locks to protect your things because Combination locks don't provide the protection you'd expect either.
Rappin' on MasterLock [YouTube]