I've had the chance to some time with ExactTarget in the last week. They're a SaaS email marketing company based on .NET with a very slick AJAX interface. Since they're an email-centric company, they use Outlook as their design center - task pane selection and folder trees on the left, interaction page on the right, etc. This parallels feedback I've heard from customers about email interfaces in the recent past.
The impressive statistic I got from our conversation was that they will send a BILLION emails this quarter - perhaps it's my ignorance of the email marketing space but I thought that was impressive.
While talking with their CTO and VP of Engineering, I learned a few things.
First, they're doing all of this on end-to-end .NET (Windows Server, SQL Server, etc.) and have achieved massive scale. Second, they're based on AJAX.NET, an elegant Open Source AJAX framework for ASP.NET (for example, to make a method asynchronous on the client, you just add an attribute: [AjaxMethod]). Furthermore, they've hand-built a number of generally useful client-side features using AJAX - a WYSIWYG editor like Writely, and due to their server-side architecture, they've been able to integrate useful data cleansing tools via SOAP interfaces. With the number of comments I get about REST >> SOAP these days, I had to ask about their use of SOAP. They said they got it for free by using ASP.NET (.asmx pages), and that the automatic harnessing of SOAP rather than raw XML was useful.
The new stuff they're coming out with - and which they demoed at Salesforce's conference recently - is significant - partly because of their use of AJAX for some sophisticated drag & drop workflow features, and partly because of the breadth of scope they can handle with their workflow engine. I can't give it all away here, but be sure to check them out when they launch the new stuff in December.
The ExactTarget team was happy overall with our technology, but were not happy with our licensing. Running on Windows and SQL Server perpetual licenses, they are charged in a way that's out of sync with their business, and while standard "failover" servers are not charged for by Microsoft, "unutilized production" servers are. Fortunately, we do have a better offering for them - I believe that the Service Provider License Agreement (SPLA) will be the solution to most of their issues.
If you're a SaaS ISV and you're using Windows, you should almost certainly be using the SPLA instead of perpetual licenses - it's pay-per-use and is a much closer fit to the SaaS operations & cashflow model.