Toptableplanner is a Web-based tool that easily enables you to arrange and create a table seating plan. You can rearrange guests and tables as much as you like and create different versions to see what works best. Just create your guest list, add some tables, and then drag and drop guests onto the plan. Once you've arranged your plan you can then output a PDF ready for printing. You can even print place cards too. This version is the first release on CNET Download.com.
I used this to arrange the seating plan for my wedding and it's great! It was really easy to import my guest list and then arrange the guests around tables. You can even print your table plan, but I was having mine done elsewhere so didn't use this feature.