For Windows: Textpad
For Macs: TextWrangler
The How Websites Work session on Thursday will involve a bunch of hands on experimentation. The only special tool you'll need for that is a good text editor. Notepad (on Windows) and TextEdit (on Macs) are basic text editors, but you can get ones that are a bit nicer, a bit more helpful, and often either free or cheap.
SO - for the workshop, you should download and install one of the text editors above. Just click on the link and you'll start the download. Ask one of the techies for help if you have trouble with the download or installation.