For the Macintosh:

SimpleText (free)
All you really need to write HTML is a plain ol' text editor like SimpleText, which you probably already have sitting on your computer somewhere. It's not fancy, but it works and it's free.

BBEdit Lite (free)
BBEdit Lite is sort of like SimpleText, but with a lot more features that make writing HTML less of a pain. The regular version is even better, but it costs US$119.

Adobe GoLive ($275)
This is a WYSIWYG editor. WYSIWYG stands for "what you see is what you get," meaning that you can arrange pictures and writing on the screen, and the program will figure out the HTML code for you. It's always better to learn the code yourself, but GoLive can make things easier.

For Windows:

WordPad (free)
All you really need to write HTML is a plain ol' text editor like WordPad, which you probably already have sitting on your computer somewhere. It's not fancy, but it works and it's free.

EditPlus ($20)
EditPlus is sort of like WordPad but with a lot more features that make writing HTML less of a pain.

Dreamweaver ($299)
If you really want to spend money on a big, fancy program to help you make Web pages, go for Dreamweaver - it's the best of the bunch.