I saw this mentioned in the Yahoo! Geotrax Group. An Online GeoTrax Track Designer, how cool is that?
NOTE: I have to set expectations correctly...as of the time of this writing none of the new sets (Big City Stuff) are yet included. But, this is definitely worth a look.
You can drag track and add other pieces onto your design, use the mouse to rotate your parts and snap them all together. And if you run out of room....simply click on your design and drag the entire track layout to a different part of the screen. (There is something you can't do in real life in your own living room...moving a track layout over even the smallest amount to avoid an obstacle can be a royal pain).