I know you really want good things that you can use in your daily work. I would like to share some of my own resources. You can add yours if you wish.
Eclipse – Eclipse is probably better known as Java IDE, but it's more important: it's an IDE framework, a tool framework, an open source project, a community, an ecosystem and a foundation.
The JetBrains IDEs (RubyMine / PHPStorm / PyCharm, etc.) are all built on the same platform and feature EXCELLENT debugging tools and a very smart code.
Sublime Text – my favorite text editor nowadays. Very fast with a rich set of add-ons available. In addition, is fully cross-platform. (Thanks to dgeneschi)
Firebug – It's a very useful tool if you ask me. You can see the code of a website with this Firefox addon.
Web developer – again, it also uses the code of the website, but unlike Firebug, you can disable CSS style sheets.
ColorZilla – you can actually know what color you want using this tool.
Using CSS – view the CSS rules actually used.
Dust-Me – Dust-Me Selectors is a development tool that scans HTML pages to look for unused CSS selector.
NoScript – Allow active content to run only from trusted sites and protect against XSS and Clickjacking attacks.
Pingdom Full Page Test – Testing the Load Time of a Web Page
Pingdom DNS Check – Test DNS Servers and Domain Name Settings
Pingdom Ping and Traceroute – Test network connectivity to a server
W3C Tagging Validation Service
net2ftp – Web-based FTP client
Web design and development tools and software:
Dreamweaver – great for coding, editing and designing a web page. It can also be integrated with Photoshop.
Web Studio 5.0
Notepad – It's FREE because you can find it on your desk.
Microsoft Expression Web
* Thanks to Sahil for this one *
http://oSCommerce.com – It's free and very good but it will not have the features like this site.
http://zencart.com – It is also open source and is created by the creators of oSCommerce but it has more features than that.
http://opencart.com – It is also good and easily customizable.
http://viart.com – He paid and free both, but I think you should check Paid, they would have some of the features.
http://xcart.com – I used it but I left it because there is no payment gateways for our country but this script is extremely good.
http://livecart.com – It has very good designs and is easily customizable from the admin panel.
http://magento.com – Function of creation of several stores (a shopping center).