I had made a small and simple marquee as addon for a client and I would like to share it with the community.
It is very simple and it works in blocks.
You just copy over the files to your stores root and install it.
Then you edit the addon b Marquee[/b] by filling your values like Text message,url link, text color etc .
Then you go over to blocks and add a new block by choosing _marquee at the Block content and placing it wherever you want.
have a try and if you find any problems please post.