Export to Excel without Excel Installed.

I need to export grid data to an excel spreadsheet.  The data will need to be formated, grouped and subtotals.  This application is VB.Net and runs from a citrix server.  I cannot install Excel on the server due to licensing costs.  The users want to be able to view the spreadsheet and save to their personal computer, and then they can open up the file in Excel if needed since they will have it installed locally.  Any suggestions.  We are currently looking some third party tools that allow this, but are trying to stay away from having to purchase a third party control.