How to: Create a tree structure w/ web controls w/out round-trips?

I am using a data grid control to present info on an ASP.NET web form. When
clicking buttons, i can show or hide columns and rows, however, this requires
a round-trip back to the server.

Is there any control (freely available or proprietary) that enables data
grids, or web table controls for that matter, to "expand" on the client-side,
without making a round trip?

Obviously this can be done w/ DHTML, but i am trying to stick w/ C# for
portability.

Thanks,