Poll: What software must you always keep handy? I don't care if it is open source, freeware, or demo, as long as its price is $0.

Neither do I care if it is for desktops, handhelds, netbooks, web based, cellphones. If it is free to use – and essential to your happiness and well-being – put it in this list.

Rules:

• Please, list only ONE application per answer, so that people can vote up the items that they prefer.
• Please do not post applications that have already been posted - instead, up-vote the existing answer.