I know you can see a simple count of how many cards of each rarity you have in game, but I like keeping track of things more than most and I also like spreadsheets more than most, so I made a little spreadsheet to keep track of which cards I have, how upgraded they are, etc.
It has two sheets. One that tracks your collection:
And one with an overview/stats:
I'm not sure if anyone else will get any use out of it, but since I made it anyway I figured I'd post it. If you're so inclined, you can make a copy of it on Google Docs at this link.
(Also sorry if this isn't the ideal forum for this; I wasn't sure which was.)