This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
Connectix's RAM Doubler ($50 street) uses the PMMU on 68030 and
68040 Macs to fool the system into believing the Mac has twice as
much memory as it actually has. RAM Doubler provides the extra
memory through a combination of compressing data in RAM, letting
applications borrow memory from other programs that aren't using
their full allotment, and storing data that would normally be in
RAM on the hard disk. RAM Doubler requires System 6.0.5 or later.
It performs as advertised, providing more RAM for your applications.
RAM Doubler does this more efficiently and with less speed penalty
than virtual memory (which can't be used at the same time as RAM
Doubler) though most Macs do slow down by 5-10% when using it. RAM
Doubler works better with multiple applications than with a single
memory hog like Photoshop. Rule of thumb: For best performance
the memory used by the system plus the largest application
partition should be less than or equal to your physical RAM size.
Ideally RAM Doubler will be transparent to your system, but
there are incompatibilities between it and some applications and
extensions. In particular you should watch out for extensions like
CopyDoubler or SpeedyFinder which can slow your system to a crawl
when they try to use all the extra RAM they think they have (but
really don't) for caching files. RAM Doubler is also incompatible
with FAXstf 3.0, UltraShield, Times Two and the various development
versions of MacsBug. It works with MacsBug 6.2.2. If you must use
a development version of MacsBug, use 6.5d4 or later and RAMDoubler
1.0.2 or later. In general if an application works with virtual
memory, it should work with RAM Doubler.
The Jump Development's Group Optimem is a more expensive ($80 street)
competing product. Optimem doesn't increase available memory like
RAM Doubler does. Instead it forces applications to make more
efficient use of the memory they have. Optimem doles out RAM to
applications only as they need it rather than allocating fixed size
partitions at startup like the Finder normally does. Go to the Finder
and look at About this Macintosh... in the Apple menu. All the light
blue (or white on a black and white monitor) space in the bar beside
each application is RAM that application has been allocated but isn't
using. Optimem makes that memory available to other applications. In
effect it forces them to share. If you have a lot of white space in
your memory bars, then Optimem can help you. If you don't then RAM
Doubler is certainly a better choice. OptiMem and RAM Doubler may be
used together. However this is going to turn RAM Doubler into little
more than another version of virtual memory since it does its RAM
compression tricks using allocated but unused space while Optimem
eliminates that space. Since Optimem is less transparent than RAM
Doubler, Optimem is incompatible with more applications. Optimem
can, however, be disabled on an application by application basis.
The one big advantage OptiMem has over RAM Doubler is that it doesn't
require a PMMU. Thus it will run on 68000 series Macs like the Plus,
SE, and Classic.