I'm always searching for new and interested WP plugins to make my sites different and/or easier to run.
I do use some of the free ones but have no problem using premium plugins.
For shopping cart sites I prefer
Shopp.
Another GFYer turned me on to
Pay With a Like.
And today I grabbed
Multi-Domains which allows a single back end to control all of your WP sites.
You'll notice 2 of them are from wpmudev.org. This site has a ton of excellent premium plugins.
What are some of your favorites?