This is actually something I wrote in 2007 but it still appears to be relevant so I figured I’d kick it out there. Since then the only major thing I can see that’s missing is the IE developer toolbar which is now a standard part of IE 8.
A web traffic monitor that hooks into the IE http stack and operates as a proxy server to allow other applications like Firefox to be monitored. Just seeing the traffic is really useful but it actually allows you to meddle with the data too! Windows only.
The error reporting of your browser can often be useful. The error reporting on Firefox is especially useful.
Web developer toolbar
A useful Firefox extension that allows you to see all sorts of information about the website.
Useful for automating testing. Allows testing in most browsers. If you copy over a blank Firefox profile before testing you can even reset the cookies and such like to make everything repeatable.
Actually this is useful for a lot of network problems. You can see what’s actually going over the wire. There is a version for most platforms.
A kickass util developed by one of the guys from l0pht. So simple yet so useful, essentially just allows you to do all those socket operations from a command line util so you don’t need to create your own tiny programs. Does tcp/udp/connecting and serving. Buffers the line you are entering so it’s more useful than telnet.
Downloads for Windows seem to have dried up but you can always grab a port written in a scripting language like Python. The *nix downloads are still alive and kicking.