RamDoubler is a software product that you can purchase which works a lot like
System 7's virtual memory. It moves things around in the system as you work
to try to make more RAM available, but since it's doing that, it will almost
certainly reduce your system performance. It also costs money, about $50, but
it will probably give you more than that amount of free RAM when you use it.
I have no direct experience with the product, so you'll have to decide for
yourself if you want to try it out.