Google Spreadsheets

Google spreadsheets is a free, online, collaborative tool that you can use to create Excel-like spreadsheets. This tool offers most of the same features and tools you would expect from Excel, but adds in the functionality of being fully online and accessible everywhere you have Internet access. Google Spreadsheets also has the added bonus of providing the ability to collaboratively edit and chat in real-time on the same document.

Some helpful links that I use:

Google Docs: Spreadsheets support forum

10 Useful Google Spreadsheet Formulas You Must Know

A quick video walkthrough showing how to create and edit Google Spreadsheets with others...