Games with level designers/modding

[TrekMD]

Micro Machines '96 on Megadrive has a track editor.

I'll have to check for that!

[sloan]

Surprised no one has mentioned NES Excitebike yet.

Let's Play Excitebike: Design mode

[TrekMD]

I didn't know that Excitebike has a level designer.  I haven't played the game yet.

[sloan]

Another series that comes to mind, although I have not played any of them, are the Roller Coaster Tycoon games.

[TL]

What about good old Railway Tycoon on the ATari ST, Amiga and PC?

Railroad Tycoon - Amiga

[RetroFox]

Mega Drive
Kid chameleon has a well designed level editor.Not tried it out yet but from the screan shots it looks like an amazing bit of software.
Theres also Sonic editors im sure many have seen

PC
Motocross Madness 2 has a built in track builder thats pretty good. Mixed choices of ramp styles ect.
Duke nukem 3D had an official map creator - great fun making strippers the size of tower blocks lol
GTA 1 and 2 both have map creation software that is really indepth and able to create whole new worlds
Midtown Madness 2 (alltime classic!) This has an amazing modding community and have map editing software and all the documentation to build 100% new maps - Its a really simple game to mod! I built a few original cities + game convertions.
Sonic Adventure DX - This game also has a large modding comunity with level creation and physics editing widely available

and go jump back to current times for a mo'

Portal 2! The map editor is so advanced and well designed. Anyone can use it! But it takes carefull planning to make a successfull map .. DEFFO worth it

All these ones are worth looking at... My personal choice would be Midtown Madness 2 as the possabilities are huge and easy to produce

Sonic Adventure, GTA, Portal 2 and Midtown madness are also great to look into just for the mass amount of levels/tracks available by the modding communities =D