iSlice

I've had mine for 5-6 years and still use it today. It's a small piece of plastic with a ceramic blade that allows you to easily open packages and cut paper. Doesn't seem like that big of a deal, but it constantly comes in handy. Especially with tough plastic packaging such as the one it comes with. I used to have to grab scissors and cut 3 of the sides off.

Source

With the iSlice, I can quickly slice the sides and it opens right now. Packages just need a quick slice down the middle of the tape and the sides open up quickly. You can use it to open mail, but I find it just isn't very efficient, so I use a traditional letter opener. It does work real well for cutting things out of paper (i.e. coupons).

This device isn't fancy or highly technical, but it is something I use almost daily and when I can't find it I get frustrated. I believe at the time I got it for around $2 shipped, but they currently sell for $6 on Amazon with free shipping.