Below is a picture of my current collection.
Disabled embedding since it's a huge picture. Be sure to scale it to 100% size so you can read the titles.
Sadly, not all of those are actual demos - The Minish Cap cartridge is just a retail copy that was bundled with a GBA. There *is* an actual demo of that floating around but people are asking absurd prices for it.
My most prized cartridges are the Pokémon distributions and the other Pokémon titles. I will actually be sending the two N64 carts off to get ROMs made of them, since nobody else has. Feel free to post any questions about these, I've tested almost all of these out myself and know what they contain.
If you've got one I don't have and would like to discuss a sale or trade, please let me know - I have a large want list, most of those have never surfaced on eBay or elsewhere.
The three DS games on the right side (above the GBA ones) are European region. If you know someone who wants these, I will part with the Nintendogs and MK7, since I have both of those in USA versions now. The Guitar Hero I think is exclusive to Europe - if not, I've never seen the USA one. Since the 3DS is region locked, I could not test the Mario Kart 7 EUR demo at all.