SpreadsheetML
This is something I stumbled on a while ago and known how to do in a basic sense. But now thanks to this StackOverflow question I also know what it's called. This will make a huge impact on my ability to use this in the future, as before I couldn't really search google or msdn to get help with it.
SpreadsheetML is just an Xml schema that Excel understands. It's important because it the best way to export Excel data from a web app. In the past your choices were Ole, COM Interop, and HTML tables. Ole never worked reliably, as it depended on a registry setting to know the folder where your document is. COM Interop doesn't scale well and isn't licensed for use with a web server. HTML tables were limited in what they could do: only a single work sheet and no formulas, for example.
SpreadsheetML scales, will work anywhere, and supports nearly all of Excel's features. You can use it when you need to send an Excel file through a text-only medium like e-mail or chat. About the only downside is that it's not supported in Excel 2000 and earlier, though my experience is that it will work on Excel 2000 in a limited way, much the same as using an HTML table.
