IGoogle's gadget directory shows the number of users and pageviews for each gadget that has at least 1000 users and 10,000 pageviews. There are many gadgets that have more than one million users, but most of them are included by default in the homepage:
* Weather (6,593,949 users / 321,210,021 pageviews)
* Date & Time (4,860,018 users / 157,754,150 pageviews)
* Google Calendar (4,193,468 users / 134,998,968 pageviews)
* Gmail (3,256,386 users / 147,062,862 pageviews),
* How to of the day (3,195,838 users / 85,462,789 pageviews)
* CNN (3,149,220 users / 90,741,669 pageviews)
The numbers seem to be an average for a week and are only approximations. But it's nice to see how popular is your gadget or feed and how many people use it.