I've been developing persistent browser based games using ASP.NET and AJAX for a little while now but an emerging technology from Microsoft called Silverlight that I believe will re-define how browser games are built for .NET developers. Silverlight is Microsoft's answer to Flash but with advantages for .NET developers in that I can program C# on the backend and have Silverlight on the front end.
My current PBBG Perenthia will be ASP.NET and AJAX but another game I have in the design phase called Aelerion will feature a Silverlight front end using (hopefully) the same game engine I wrote for Perenthia.
6/30/2007