Two older Macs (G3 / Intel) need their drives erased. No install discs

I have two older Macs in which I want to erase the hard drive. Secure-erase is a plus, but not a show-stopper at this point. I don't have any Mac OS X discs so I can't use disc utility.

What I'd really like to do is download or buy and download a utility that can create bootable discs so I can boot each computer and erase the hard drives.

Any suggestions on utilities?

Do you have a current Mac with a Firewire port? You can attach the old Macs¹ to the new one via Firewire, and boot them using Target Disk Mode, and use Disk Utility on the new Mac to wipe the drives (they'll appear as external drives).

¹ Note that the earliest G3 iMacs didn't have Firewire, and thus can't use this feature. The G3 with slot loading drive (released in Octoboer 1999) and later will work.

An alternative method is to use a program called TrueCrypt to do a whole disk encryption, then boot into Linux (live CD or USB) and format your drive with whatever you want - or do as @robmathers suggested with Target Disk Mode and erase the drive. This way the data would not be recoverable, and you wouldn't have to use a live CD to format.