A RAM disk could be SOOO useful if you actually know what it does.
You know how using virtual memory, you can use hard disk space to emulate memory? A RAM disk does just the opposite. It takes memory and makes a virtual hard drive.
Obviously, the disadvantages that virtual memory provides are flip-flopped into advantages for a RAM disk. While virtual memory is very slow because a hard drive is slow compared to regular memory, a RAM disk is FAST compared to a regular hard drive for the same reason.
So you allocate however much memory to a RAM disk, and then you basically use it like a hard drive. The only difference is it's much faster, so if you want to run applications DIRECTLY from memory, you just copy it to the RAM disk, and then launch it from there. The most useful thing, though, is that you can use it for things that you really use often, like SETI@home. Instead of having SETI@home write the data to the hard drive, you can dramatically speed up that time by using a RAM disk, and trick SETI@home into writing there by using an alias.
Of course RAM disks do have disadvantages. If you zap your PRAM or experience a crash, I believe all of the data in your RAM disk is zapped.
This makes it unreliable. However, whenever you turn off your computer, the contents are actually paged back to the hard disk, and are paged back in on startup (at least this is how it worked in OS 9). Another disadvantage is that you can't use the allocated memory for normal memory usage -- it is basically dedicated memory for a virtual hard disk (unless OS X could provide a dynamic RAM disk, which would be frickin' awesome).
I'm not sure if UNIX has the ability to do a RAM disk, but it would be awesome if we saw this functionality back in OS X 10.2.
