LaunchBar would be my vote for "Indispensable Tool." It's a little utility provides access to everything (applications, urls, email addresses, shell scripts, etc.) from the keyboard. Apple-Space+"GO" opens up Google in OmniWeb for me, but you can train it to open whatever you wish, easily (built-in adaptive search algorithm: standard). ObjectiveDevelopment creates some other ultra-useful applications, including an easy-to-use SAMBA client called Sharity.