I haven't come accross any web-based layout programs, but there are two freeware titles that you can download and install on your computer.
- Atlas released their layout program, RTS, as Freeware. You have to register with an e-mail address to access the download, but you could use a temporary 'junk' address. Once installed the program requires no further interaction with Atlas.
- There is also XTrkCad. It took me a bit of searching to find an active download link; it appears the program was abandoned by one group and another group picked it up.
XTrkCad contains libraries for many different scales and manufacturers. RTS is a bit easier to learn and use, but only contains library files for their own track sets.
There are many commercial programs too.