Out of the games I have, this is what I've downloaded.
-Mario vs Donkey Kong 2 - there are a handful of Nintendo-created DLC puzzles
All of the 6 puzzles released.
-Mario vs Donkey Kong: Mini-Land Madness - TONS of DLC puzzles, need more than one cart to get them all
-Mario vs Donkey Kong: Minis March Again (DSiWare) - lots of DLC puzzles here too
-Picross DS - download puzzles from 10 older Picross games, requires 10 carts to get them all (I actually ordered 9 extra copies of the game because I love Picross / am insane)
Most likely won't be able to get all of the content, due to the insane amount of carts needing purchase in such a short time. (Seriously though, Nintendo: First time you offer NP games in years and people can't even own them all)
-Professor Layton (all) - weekly puzzles
-WarioWare DIY - downloadable puzzles, requires 2 carts to get them all (my 2nd cart is on the way)
Spread across two titles, (DIY and DIY Showcase). Layton 4 came in today, have all the puzzles.
These are the titles that I think I can get in a short amount of time, but will need questions answered on some of them:
Stuff I still need to grab DLC for:
-Advance Wars: Days of Ruin - TONS of downloadable maps/missions
-Dragon Quest IX - downloadable maps, not sure if these are still available...
-Daigasso Band Brothers DX/Jam with the Band - TONS of downloadable songs
-Fossil Fighters Champions - apparently there are downloadable fossils but I haven't figured out how to get them yet
-Heroes of Mana - apparently there are downloadable missions?
-Professor Layton and the Last Spector - apparently there is some kind of DLC in the 'Layton Life' mode
-Picross 3D - weekly puzzle packs and challenge packs, requires 2 carts to get them all
-Pokemon Ranger: Guardian Signs - download missions
-Pokemon Ranger: Shadows of Almia - download missions
Advance Wars: Can all the missions fit on one cart? Is it possible to download them all near the start, like the Layton titles?
DQ9: What specifically are the codes for the DLC quests/codes? I looked around and there's only catch-all "Every armor/clothes/shield/etc" codes and one for the "Legacy Dungeons"
Fossil Fighters: Any more info on this? When to get the DLC, what do I need to do in-game?
Mana: Same as above.
Layton: Is there a AR code for the extra friends in London Life, like how there is one for the weekly puzzles?
Picross 3D: Are the puzzles worthwile?
Ranger Guardian/Shadows: Weren't those missions timed events that are gone now? Or is there an AR code to "Patch" them in?
For ones not covered on the list:
Fire Emblem DS: That shop exclusive sword, how much in-game currency does it need? (I'm assuming you pay through that.)
Castlevania DoS (I think?): Did that item shop Wi-Fi thing do anything? As in, can I play the game full to completion without that?
The Last Story: What specifically do you have to do in the online mode to get the exclusive sword/whatever else?
And finally:
Are there any other games the thread has missed that have exclusive downloadable content?