J# Browser Controls (~Java Applets) are the closest you get web client side
logic in the .net framework.

I suggest you make a "normal" .net Windows Application (you run in the
client IE context), that makes use of web services.

Lars-Inge T?nnessen
www.larsinge.com

In addition to JBC or J# Browser Controls, you can host .NET applications
in a web page with the object tag.  For example, a .NET UserControl written
in J# can run embedded in IE just as one written in other .NET languages.  
The difference between this and the "normal" .net windows application
mentioned is that an object tag embedded application runs in the same
process as the browser, while a top-level window application runs in a
separate host process that retains an Internet execution context for
security.

Here are some links:

Here are the late-breaking J# quickstarts:
http://samples.gotdotnet.com/quickstart/latebreaking/default.aspx?lang=VJS

Sample of embedded .NET application in IE browser.
http://msdn.microsoft.com/library/en-us/cpguide/html/cpconconfiguringieappli
cations.asp

I am looking for a way to do complex client-side logic.