Is there an external tool or plugin which can be integrated with Marketo to track or reverse lookup the IP details of a anonymous lead/ know lead to get more information about the lead's company and demographics?
If you search LaunchPoint, there are several. For a known lead, you'll want to enhance based on the email address together with the IP (and, of course, if someone signs up with their work address from home, you don't want to treat their ISP information as if it's their company).