There are a number of tools that we like to use at Izolo.

A number of them help a great deal with understanding what is really going on with web services. Some speed development and others provide the necessary error correction to produce great, international sites.

For validating XHTML:
validator.w3.org

For validating CSS:
jigsaw.w3.org/css-validator

For checking file sizes and transfer optimality:
www.websiteoptimization.com

For international speed testing:
www.alertsite.com

Some excellent Firefox extensions:

Web-developer toolbar

View formatted source

A window into AJAX requests:
Firebug

For filling a page with words before the real content is ready:
Lorem Ipsum Generator

A fantastic talk on UI Design from Oscon ‘06.

Hopefully these help you understand and develop web sites.