1. Charles is my all time favorite while working with amfphp / http services.
http://www.charlesproxy.com/download/
2. WireShark comes as my second choice; it really helpful to see inside network packets. It's available free of charge
http://www.wireshark.org/download.html
3. While working with html UI parts or css or ajax; firebug is very useful addon for firefox. It's power lies in right click menu "Inspect Element"
http://getfirebug.com/
4. Firefox extension for taking screenshots (with scrolled content)
http://www.screengrab.org/
Read more: Codeproject