I thought it would be cool to write a macro that logged users that opened a workbook. Well I did, however after doing a search it turned up an example at Jwalks site that is much cleaner. It is also a perfect example if you are looking for a no-frills way to add a line to a text file when the need arises.

Link:
http://www.j-walk.com/ss/excel/tips/tip95.htm