Input Director: use one keyboard & mouse for two computers

Input Director is a software solution used to control two computers using one keyboard and mouse. It make VNC, Remote Desktop or KVM obsolete. Read more and get it here or here.

Extended Statusbar: the easiest performance testing tool

A Statusbar with speed, percentage, time and loaded size (similar to Opera's one). You can get it here

Bottom Error Console: shows Firefox Error Console on the bottom of the page

Bottom Error Console splits window and display error console on the down side. You can toggle the presence of the display with the statusbar button or shortcut key (Ctrl + Shift + B). You can get it here