Full disclosure: I received these stickers for free in a contest.
Let's face it, the Intellivision cartridges were pretty plain. They only have an end label with very little embellishment. You may have seen these already, but a user over on AtariAge has created some really nice screenshot stickers that fit perfectly on the blank front of Intellivision carts. They really look good, and it's kind of fun to apply them. The carts end up looking a lot better -- they remind me of the Activision cart design. They are glossy and have a very professional look/feel.
Here are the two sets i received (I think there are a total of 3 to buy).
And here are a few of my loose carts with stickers applied.
If interested, look up "Age of Atarius" on Facebook. Or PM me and I can hook you up with the guy who makes them. They are priced very fairly IMO.