Several times daily
I have three clocks set to different time zones, and the Sun Clock which keeps track of night and day around the world. I also often use the Calendar.
Unit Converter, mainly for currency. I also use it to convert from the archaic imperial units still used by the USA and metric units used by most of the rest of the word.
The five day weather forecast is sometimes useful.
Right now I am listening to BBC Radio 2 using Phantom Gorilla's BBC Radio widget.
I often use the iTunes timer to put the computer to sleep after I have dozed off to the strains of music from one source or another, often the above mentioned Radio 2.
Then there is iStat pro to keep track of where the HDD storage is at. It also shows that the 5 GB of RAM is more than adequate for my needs at present.
I do open and use other widgets from time to time.
Dashboard opens at a click of the scroll wheel on my Logitech mouse. I would not like to see it go in some future iteration of OSX.