I haven't played Space-Jerusalem / Natural20 (homebrew D20 system), but there is one idea I like alot. That system uses cards as equipment, special skills etc. for characters.
Usually in rpg's you write down what ever equipment, weapons, spells, special skills etc. your character has. It works ok, they all are written down on character sheet. But sometimes there are problems. You might have title for skill or spell your character knows, but there is not enough space to write description how it works and what it does. Before you learn what they do, you might have to refer to rulebook quite often.
That can slow down action in games.
Currently I am Dungeons Mastering D&D 3. and my player's character is a sorceress. So obviously she has spells. She is third level now, but at this point she already has 10 individual spells. Ofcourse they are written down on character sheet, but there isn't that much space for spell descriptions.
One way would be to write down spells statistics on separate paper, but I don't like the idea. I remember when I started to play and run Vampire: the Masquerade, I did write down discipline's name and below it what that discipline can do in different levels what my character knew. Doing that I didn't have to refer to rulebook every now and then figuring out what I have to roll to success, how much blood or willpower it consumed and so on.
When my player's sorceress in D&D 3. started to learn more spells than few, I decided to download Magic Set Editor 2 (thanks for the idea, Space Jerusalem). Now when sorceress character learns new spells, I just check out the statistics needed in the game and write them on blank card template, google a picture to paste on it and save. I have a card for spell you can quickly refer during game without need to open rulebook!
We do use computers in gaming as a tool. So it's easy to shuffle between the cards with image viewer. I could also print them ofcourse.
Now I have a ambitious plan. To make card of every single spell in PHB. That's alot of work, but when it's done, it will be so neat. I have every spell in game made as an easy-to-read card!
I am also planning to make cards of weapons. Googling a picture is quite easy and it is easy to write down weapon's statistcs. And if I print them, player can actually have a deck of items and spells her character knows! No more writing own weapons and spells and rules for them, you just take your card deck and are ready to start gaming. Just put down next to character sheet weapons and items, and when deciding what spell to use, just shuffle the spells deck. Fast and easy, and you have good visual images of stuffs.
Cards are fast to do, so if character obtains something, it takes minute or two to make a card of it. And good part is also, that you can quite easily manage character's equipment list. What items your character is carrying at the moment, you take aside from the deck. No need to remember or mark on character sheet what you are carrying with and what are in your stock or haven.
Currently I am using this system only with D&D 3, as I started to making spell cards after started playing D&D. But I am planning to make cards for other games aswell. Even if I already quite well know how V:tM disciplines work, it would be neat to make discipline cards also! And bloodpool? You can use markers or die showing how much you have them, as the amount of bloodpoints is constantly changing. But I can just make bloodpoint cards (picture of bloody number maybe) what shows how many bloodpoints character currently has. So during game no erasing and marking bloodpool, but shuffling cards how many you got left (needs atleast 10 cards, from bloody numbers from 1 to 10). Advantage of using this system is that you don't accidentally turn your bloodpool die, you don't need extra space for two piles of counters (current bloodpool and extra bloodpoints to add after feeding) and you don't have to constantly erase your character sheet or you don't need extra piece of paper to mark them down.
Also for V:tM it could be cool to make weapon cards. Easy to shuffle cards with weapon statistics, and as V:tM uses real weapons, finding neat pictures is easy.
I know that making cards doesn't necessarily improve gaming at all, but I think it is a neat tool and can make gaming a bit easier. And usually my games are more about story and drama, so these cards could add a bit more "gaming" part into game.
If I recall right, Space-Jerusalem does rely on cards alot. It can be ofcourse be played without cards using traditional character sheet where you write stuff down.
So, I like the card idea. They are fun to make, fast to make and can make gaming a bit faster and easier. But what do you think? Are additional cards to games just too much more things to handle? Or can they even distract playing in some way?