Excel Package is an open-source (GPL) component for reading/writing Excel 2007 files. I used it on a small project, and the API is straightforward. Works with XLSX only (Excel 200&), not with XLS.
The source code also seems well-organized and easy to get around (if you need to expand functionality or fix minor issues as I did).
At first, I tried the ADO.Net (Excel connection string) approach, but it was fraught with nasty hacks -- for instance if second row contains a number, it will return ints for all fields in the column below and quietly drop any data that doesn't fit.
