There are many articles on the internet that provides you with formulas on how to convert a 15 digit Salesforce ID to 18 digit. However when I was looking for online tool to actually do this conversion, I couldn't find one. So I created my own tool that allows me to do just that. If anyone else is interested in using this tool I've made it available at:
http://www.magentrix.com/aspx/ConvertSFID
Happy Converting