Depends on what you are collecting, you need to know your area of collecting.
There's a lot of variation depending on region & system, especially the Nintendo stuff, like SFC have a small green piece of paper in them (a warning slip) & some have a piece of card showing game controls eg: super donkey kong, some like Cotton 100% & Final fight guy should contain a music CD.
Then there's the RPG's that often contain maps.
I used to collect NES games & a lot of them would have posters included like GB & snes games, this depended on the publisher.
(obviously the Nintendo carts should also be in a clear plastic bag for 100% completeness)
Sorry, I don't think I've helped you much