I recently posted an article about LinqToExcel, a fantastic library I discovered while trying to solve a problem for a work-related project. The premise was that I wanted a library I could use to read data from MS Excel files which may be uploaded to a website, and LinqToExcel seemed to fit the bill nicely. Until, that is, I went to deploy to Windows Azure Websites (a requirement of the project in question).
Enter ExcelDataReader, an Open Source project which facilitates reading data from Excel spreadsheets by working directly with the Binary Excel file (.xls files), and/or abstracting the OpenXml library to provide a more familiar Data Access paradigm (.xlsx files).
Also note that ExcelDataReader is Cross-Platform and will run on Windows, as well as (under Mono) OSX, and Linux.