There is a new online tool available: the online spreadsheet merger. This simple tool allows you to merge multiple spreadsheets into one, select relevant columns and convert plain-text spreadsheets (.csv/.txt) into Excel format (.xls) and vice versa.
This tool has been created mostly as a convenience for users of OpenSesame, the graphical experiment builder, which outputs data as separate .csv files per subject. The online spreadsheet merger has been created to accommodate users who prefer to have all participant data in a single spreadsheet.