Webmaster resources

[Cedik]

I searched the forum for a similar thread but couldn't find one so I'm opening this one.
I was wondering if you know any sites that would prove useful to webmasters.
This link is great, as it provides many on-line tools: http://www.iwebtool.com/tools/
I also use a lot: http://tools.summitmedia.co.uk/spider/ It's a spider simulator. It helped me improve my site in terms of SEO.
And this one, http://validator.w3.org/ , I think it's know by anyone.
These are the only good sites that I know though...
If you know others, please post them here