Recommended links

Office Excel 2007 Assistance
An online application from Microsoft, for learning the locations of the 2003 buttons and menus in the 2007 version.

The Official site
Excel introduction, overview and description, from Microsoft.

Excel Tips
The most comprehensive Excel tips site in the internet.

Excelforum
One of the best Excel forums there is.

Mr. Excel
Tips and Solutions

A basic tutorial
From the University of South Dakota.

Vertex42 - Templates and links
A huge collection of Excel templates and calculators, as well as directory with numerous relevant links. (Check their tutorial directory here.)

The wikipedia description
The description of Excel in the free encyclopedia.