Rasputin - description from Softdepia.com:
Do you get annoyed when your Internet connection gets closed by your Internet service provider because of a lack of activity? This application simulates network activity to keep your Internet connection alive. This version adds an option for Windows Dial-Up Networking (DUN) users to automatically disable signaling while not connected. This version features improved signal effectiveness.
Rasputin is a "Free to try" program. You can download and try it for an evaluation period.