If you have them installed on different partitions, you can select which one to boot from by holding alt/option when you boot (on new-world machines). Unfortunately, that doesn't work for different versions on the same partition, but apart from OS 9 and OS X, that's not really practical anyway.

Click to expand...

In earlier Mac OSs you easily can choose to boot from a distinct drive with the help of FWB Harddisk Toolkit. It can be configured to address the startup volume during the boot processs with a key combo. It might not work for multiple partitions on the same drive, but this would be a nice project for the code warrior department, would it not ?