Hi;I came to weblogs after a long time, saw your post and thought to put something. Well! I believe its actually hard to find a free exactly excel type AJAX enabled component. You can download and use http://trimpath.com/project/wiki/TrimSpreadsheet a totally java script enabled spreadsheet. All work would be done at client side without any post back. and if you wanna add out-of-band requests to save your data or some additional features please see this two part serious excellent article on ibm http://www.ibm.com/developerworks/web/library/wa-ajaxintro1.html
You might be able to design a .net enabled ajax server component by mixing these two things. Hope this helps.
Welcome back "developer"
Thanks for the links; we started using Infragistics' Ultra Web Grid; which provides similar functionality...