Notes - a blast from the past,
when "nobody ever got fired for buying IBM" was the maxim by which IT senior management lived at my former employer. I wasted many an hour in Notes reading unimportant emails from management and colleagues. It is where I first learned to hate the ability to create groups of users and blast everything to everybody, whether it affected them or not.
Lotus Notes obviously wasn't the only email system with the ability to bring productivity to a grinding halt, but it is the one I was subjected to in my daily endeavor to actually get something done.