Software

Good stuff to download & use

Web browsers

Mozilla Firefox
http://www.getfirefox.com

Firefox Extensions

Web Developer
https://addons.mozilla.org/firefox/60/

Total Validator
https://addons.mozilla.org/firefox/2318/

IE Tab
https://addons.mozilla.org/firefox/1419/

X-Ray
https://addons.mozilla.org/firefox/1802/

View Source Chart
https://addons.mozilla.org/firefox/655/

Live HTTP Headers
https://addons.mozilla.org/firefox/3829/

FireBug
https://addons.mozilla.org/firefox/1843/

Text Editors

jEdit
http://www.jedit.org/index.php?page=download (get the latest, not the stable, version)

Cheatsheets

Excellent quick reference guides. Download these files & print them out.

X/HTML

HTML Cheat Sheet
http://lesliefranke.com/sandbox/ref/htmlcheatsheet.pdf

Cheat Sheet v. 1.03
http://www.cdburnerxp.se/htmlcheatsheet.pdf

HTML Character Entities

HTML Character Entities Cheat Sheet
http://www.ilovejackdaniels.com/cheat-sheets/html-character-entities-cheat-sheet/

CSS

CSS Cheat Sheet
http://www.ilovejackdaniels.com/cheat-sheets/css-cheat-sheet/

References

(X)HTML

XHTML 1.0 reference with examples
http://zvon.org/xxl/xhtmlReference/Output/index.html

CSS

CSS 2 reference with examples
http://www.zvon.org/xxl/CSS2Reference/Output/index.html

Validation

Check your code with these free tools

HTML

W3C MarkUp Validation Service
http://validator.w3.org

CSS

W3C CSS Validation Service
http://jigsaw.w3.org/css-validator/