Here's my favorite; a Dritz seam ripper from Walmart (not affiliated, they just happen to have a good pic of it):
http://www.walmart.com/ip/Soft-Grip-...ipper/19757837
The reason I like it is because it doesn't roll off the table when I set it down, and it fits perfectly in my hand.
I have other seam rippers including the Clover, but I'm in love with the Dritz ripper and as a celebration point, it's not the most expensive tool in my shed, which I can't often say of other tools I've purchased over the years!
