AjaxWidgets announced 100% Mono-compatible Controls
The new version of Gaia Ajax Widgets now is shipping with full support for Mono ASP.NET 2.0.
If you were considering an ASP.NET control library for use in a cross-platform fashion, these guys offer a very nice suite of controls, and they will support you in Mono and Linux.
In addition to the controls, they provide MonoDevelop solution files and ready-to-run components. In addition to supporting the "big browsers", AjaxWidgets works just fine with Opera.
If you are an open source project, you can use those controls for free. For commercial projects they offer a commercial version that is very reasonably priced.