Most open source software involved with web design is for the back-end, like databases, web servers, and scripting languages. However, there are some powerful tools for designing and crafting websites themselves.
These include the following programs:
- Bluefish - HTML and Scripting IDE
- The GIMP - Graphical Image Manipulation Program
- mozilla-cascades - Stylesheet editor
- Screem - Gnome website development toolkit
- Gphpedit - scripting and html editor
I've worked with Bluefish a bunch and liked it a lot, but the version I used was still a little buggy.