Must use tools and libs for web applications

1- Firefox, Firebug (No need to debug with alerts, can also track http requests/response). FirePhp for php users provides additional logging features for server side script.

1- jquery (script your apps with it, its really powerful)

2- jquery ui (change ui themes in minutes, even provide theme change feature to website users)

3- jqgrid (a javascript + ajax grid library, very powerful, search, filter, paging etc)