Here is a simple paint canvas done in javascript and HTML:
Web Speaks: Create Simple Paint Canvas using JavaScript
If this isn't exactly what you want, or you seek alternatives, try googling
simple paint in javascript.
As for saving it from a javascript/html display to a server... that may be a bit tricky, as Javascript and HTML are client-side code, but a server (obviously) uses server-side code, like PHP.
There are almost always workarounds, though; especially when there are other scripts that do it
.
After a few minutes of research...
Server-side Javascript library (I did not read much in to this at all, no idea if it will prove useful):
http://ejohn.org/blog/server-side-javascript-with-jaxer/
You could also try using CGI, ASP, or Flash instead of Javascript. The script you posted:
http://www.pendo.nl/demos/handtekening/index.html is written in Shockwave, which is essentially Flash. (I don't want to get in to the gritty differences, heh...)
But, why not just buy the script you posted? It's just under 11Euros. You would probably be saving a lot more time, and thus money. (Do you want to spend 20 hours finding/building one, or one to two hours money's worth on a nicely built product?)