1) Adventure Construction Set (Electronic Arts, 1987)
ACS is a remarkable little toy that you can waste hours of time on. It allows you to create mini "adventures" that are more like RPGs than classic adventure games. With ACS you can create maps of worlds and dungeons, create characters and monsters, create and use normal and magical items, create in-game text boxes, and all sorts of other fun little details.
One of the best features of ACS is the map-making capabilities. You can create complex dungeons in which there are stairs, hidden doors, trapdoors, magical portals to other places on the map, place objects (including user-created objects), etc. There are different kinds of terrain to choose from, such as forests, mountains, lakes, rivers, and other standard kinds of terrain. ACS is one of the best tools I've seen for mapping Interactive Fiction games, but sadly this does not seem to be widely recognized by the IF community at large.
Some people have claimed that it can't be used under DOSBox, but this is not true because I discovered a trick to make it work. ACS will ask you to insert a "game disk" in a floppy drive, but the way to get around this is to mount a directory as a floppy in DOSBox, then when ACS asks you to insert one, switch to your main OS and temporarily remove the files from the "floppy's" directory, then restore the files once ACS has created the disk. Overall, ACS is quite dated, though. It has a limited 4-color palette, and you need to greatly reduce the speed of the game to play it because it is not completely turn-based: your time can run out much too quickly to perform an action during gameplay. Still, this is one of the earliest game-creation systems that didn't require any programming knowledge, and it packs a whole lot of fun in a small package.
2) Forgotten Realms Unlimited Adventures, (SSI/Micromagic, 1993)
Unlimited Adventures, or FRUA, really lives up to its name. It is one of the best RPG creation tools ever made, IMHO. Unlimited Adventures uses AD&D mechanics, which was officially licensed to Strategic Simulations Inc (SSI). SSI then created many classic "Gold Box" AD&D RPGs, such as Pool of Radiance and Champions of Krynn. This was a great move in gaming history, since SSI is without a doubt one of the best, if not the best strategy companies in the history of computer gaming. There are other RPG creation systems, most notably Bard's Tale Construction Set, but Unlimited Adventures is not only a quality product, but it is incredibly easy to use, and you can create your own "Gold Box" RPGs with it.
With FRUA, you can create your own world and dungeon maps, use your own images for character pictures, customize many features of the game environment, and of course you can design and play games with it. There is still a hardcore community of FRUA users and many game utilities, patches, artwork, and user-created modules can be found at the UA File Archive. FRUA is still being kept alive through a remake project called Dungeon Craft, which allows people to create and play FRUA games for free. But while Dungeon Craft works in Windows and has some significant improvements over SSI's creation kit, there's something really special about the original to me because it is closer to the spirit of the old Gold Box classics.
3) OHR.RPG.CE (Hamster Republic, 2001)
Hamster Republic RPG Creation Engine was never a commercial product, but it is still one of the best RPG creation tools around. While it isn't quite as old as ACS and FRUA, the original DOS version written in QuickBasic has that old skool feel to it. OHR.RPG.CE takes more work and patience than either ACS or FRUA, but the results can be very rewarding. There is also more portability with OHR.RPG.CE, because you can play the user-created games in Windows, DOS, and Linux.
There's also a thriving community built up around this nifty little game creator, and many user-created games can be downloaded for it. While I personally don't enjoy it as much as I do FRUA and ACS, the fact that it is a versatile freeware game creator makes it worth mentioning.
Now go forth and make some games!