1) Client Side: Chrome.
Lo and behold the Developer tools in Chrome KickAss. Firebug was good, Chrome Dev tools are fantastic. The javascript console even does code complete.
2) Server Side: HTTP Client
Nice, free, graphical tool to inspect HTTP packets. Send posts, gets, heads, etc… to any url and inspect the request and response with nice syntax highlighting. Makes it easy to see what your (or any) server is up to…
