NatureServe and The Nature Conservancy recently produced a tool to assess the invasiveness of non-native plant species. From the main web page, you can download .pdf versions of the documents, including instructions, data forms, and example assessments. They also released the ranked results of their own assessments (300+ down, only 3200 to go!).
Thanks to a member of the Yahoo! group ma-eppc for posting about this great new tool.