Set up flash drive with multiple OS X Installers
I have an external hard drive with several installers on it. All you need to do is make several partitions, each large enough for the disc you're imaging. (My drive has 10.4 Tiger PPC, 10.5 Leopard PPC, 10.5 Leopard Intel, and 1.6 Snow Leopard on it, and I have a Lion USB flash drive.)

I made the images using Disk Utility's Restore feature. Select the source disc (in the optical drive), choose the "Restore" tab, and make sure the selected disc is in the "Source:" field. Drag the intended destination partition to the "Destination:" field, hit the "Restore" button, and wait.

There are several caveats to keep in mind:


A FireWire disk is preferred to a USB disk, as not all Macs can boot (or boot easily) from USB. 
Pay attention to the partition scheme on your hard drive: PowerPC Macs boot from Apple Partition Map and Intel Macs can boot from APM and GUID, so APM is preferred if you intend to repair PowerPC machines. You might want to have two drives, one for PPC and one for Intel Macs.


One good thing about having the installers on a drive is that hard drives are much faster than optical discs.