OK I'm confused. As far as I can figure out, after you finish the final mission, the game is over. You cannot continue on any sort of freeplay to finish your achievement hunting, etc.

If there really is no freeplay, why on earth is one of the achievements "collect all collector cards on a single playthrough"? That eludes to the fact that you can continue collecting these cards on subsequent playthroughs to finish other achievements (e.g. the one for the card minigame).

I don't understand. Please can someone inform me? I am not playing through this entire game again to finish up some of my achievements. I'm thoroughly disappointed.

I was disappointed with the lack of free play after the conclusion also. Free play is something that game-makers have allowed recently. Sadly, not so with Rage. Perhaps that will change with DLC content.

Yeah, unfortunately there is no freeplay mode after you beat the game. I ran into the same problem you did. It's ridiculous having an open world mission type game and not allowing players to go back to finish up missions and stuff. I thought after I beat the game I could go back and collect the rest of the cards but no such luck. And what svcks even worse is that even if you reload from the point BEFORE you completed the final mission, some of the areas where there are cards hidden are blocked off. So it looks like the next time I play through the game (which will probably be a while from now), I will have to sit down with the card list right beside me so I don't miss one and then am forced to replay the whole game again just to find a card that I might have missed.

Very disappointing. I was really looking forward to 100% completing this game since it was so much fun. Looks like I won't be able to do that unless I replay the entire game again.

You need to reload a previous save (if you have one) to keep playing.
Otherwise there's multiplayer and co-op... and some DLC in the future.