Always Be Testing: The Complete Guide to Google Website Optimizer (Paperback)
by Bryan Eisenberg , John Quarto-vonTivadar

Bryan Eisenberg details every aspect of the webpage that can be tested. There is a chapter by chapter .. blow by blow account of what can be tested. All the chapters mention additional online resources, you can get more detail from.
Testingtoolbox.com is the companion site for this book.This site should have listed all online resources mentioned in the book. It’s pretty painful to type in something like http://www.grokdotcom/11/03/2008/the_concerned_chapter

Here’s a list of some of the tools mentioned:
A. Test how your page looks on different browsers at http://www.browsercam.com/
BrowserCam’s Screen Capture Service lets you submit single or multiple URL’s, choose the browsers and operating systems you want to see, and screen captures of your webpage are loaded in the different browsers and operating systems you selected.

This is how pune360.com looks in different browsers.

B. Lynx Viewer http://delorie.com/web/lynxview.html
This service allows web authors to see what their pages will look like (sort of) when viewed with Lynx, a text-mode web browser.
This is how pune360.com showed on lynx

C. http://websiteoptimization.com/services/analyze/
Here’s what website optimization has to say about Pune360.com.

D. http://tools.dynamicdrive.com/imageoptimizer/
Image Optimizer lets you easily optimize your gifs, animated gifs, jpgs, and pngs, so they load as fast as possible on your site. Furthermore, you can easily convert from one image type to another. Upload Size limit: 300 kB

Recommended books:
Speed up your Site: Website Optimization Andrew King
High Performance Websites: Essential Knowledge for Front End Engineers Steve Sounders

E. Search tools: http://www.searchtools.com/tools/tools.html

An excellent example of a site search requirements, analysis, selection and installation process is available at the University of Pennsylvania’s web team area.
http://www.upenn.edu/computing/web/webteam/rnd/search.html

F. http://readability.info/
Curious about how complex your documents or Web pages are to read? You don’t have to get a team of experts to generate your readability score: you can just use readability.info to analyze the characteristics of your writing and ascertain a multitude of readability scores. By comparing the readability score of different documents (or Web pages) you can better hone your writing and make sure that you aren’t creating overly complex sentences and paragraphs for your audience.
Details on scores: http://readability.info/info.shtml

G. http://www.vischeck.com/vischeck/vischeckURL.php
Vischeck is a way of showing you what things look like to someone who is color blind

H. Customer Review solutions http://www.powerreviews.com/

I. Google Analytics http://blog.dogster.com/2008/09/25/get-more-out-of-google-analytics

J. http://uitest.com/en/specials/ (list of online tools)

Even though the book says that it is the complete guide to Google Website Optimizer, there are only 3 chapters in book of 46 chapters that talks about Google Optimizer. The other chapters are related to testing per se.
Either ways a very well written book. A must read, for anyone wanting to test their website.