Which are the OS X Dashboard Widgets you can’t live without

I don't use widgets that much, but one particular widget makes the dashboard useful to me. That's iStat

iStat Pro is a highly configurable
widget that lets you monitor every
aspect of your Mac, including CPU,
memory, disks, network, battery,
temperatures, fans, load & uptime and
processes. iStat Pro can do it all.

And it's FREE

Do you have one outstanding widget to share? How do you use it?
Please follow these simple rules:

1. Please search for duplicate answers before posting. You can search
   this question like this: inquestion:this iStat Pro
2. Limit one dashboard widget per answer.
3. Add a link to the website in the name of the application if possible (no direct downloads).