HOW TO: Implement Multiple Forms in ASP.NET

Multiple Forms and Non-PostBack Forms by Paul Wilson (ASPAlliance.com) - provides a custom control solution (a real server form) which supports WebControls, ViewState, and Events, as well as Action to Post to other pages/sites. The only limitation is that Validation controls are not supported but can co-exist with a single regular ASP.NET server form. Update (Sep 3, 2004): Open-Source WilsonWebForm is available on SourceForge.
Form-based Programming in ASP.NET by Dino Esposito (MSDN Magazine) - also provides some insights on implementing multiple forms in ASP.NET i.e. using multiple form tags, using user controls to host logical forms, and using a custom form control.