An alternative is Gaia Ajax, a little known commercially available library that does what Microsoft was about to do years ago -- that is, offer a set of ASP.NET server controls that have the same programming interface as AJAX. Gaia Ajax offers relatively low-level controls such as Button and TextBox that are, for the most part, derived from original ASP.NET controls but enriched with the unique capability of supporting asynchronous AJAX postbacks instead of plain postbacks.
Moreover, Gaia Ajax encourages developeers to continue using the familiar programming style of ASP.NET Web Forms. AJAX comes free with the surrounding library. Had Microsoft done ASP.NET AJAX this way, we could have upgraded our applications to the next version of ASP.NET gaining AJAX capabilities almost for free.
So what's the difference between Gaia Ajax and ASP.NET-style partial rendering? Both preserve the classic ASP.NET programming model. With Gaia Ajax you don't need to use the ScriptManager control, nor wrap your controls in a special updateable panel. Neither does partial rendering allow multiple simultaneous calls. If two calls occur at the same time, then the oldest is killed to make room for the newest. Gaia Ajax, instead, employs a queue transparently. Finally, with Gaia the update of the page is faster because it is smarter. While partial rendering refreshes a portion of the page by setting the innerHTML property to a new HTML string, Gaia Ajax controls keeps in sync the server-side tree of controls with the DOM-level tree of controls. Any updates, in the end, are applied by modifying the individual element in the client DOM tree without erasing and replacing the entire tree.
What makes Gaia Ajax worth considering is just its underlying architecture -- it lets you avoid additional learning curves by allowing you to do your AJAX-based development in an ASP.NET familiar way. In other words, you keep on writing ASP.NET apps, except with Gaia Ajax they're automatically AJAX enabled.