by 
Modder mntorankusu has created a software tool which allows you to dump the content of N64 carts using nothing but a flash cart.
N64 SwapDumper is "inspired by and based on sharksaver64 by @Jhynjhiruu, @ppcasm and @Modman" and allows you to dump saves, ROM data, or both.
"It only requires a flash cart, no specialised dumping hardware," explains the developer. "You run the application on a flash cart with an SD card (only tested on SummerCart so far), swap in the cart that you want to dump, and it copies the largest chunk it can into RAM. Then you swap the flash cart back in, and it saves that chunk to the SD card. You do this back and forth until the entire ROM is saved to the SD card."
The tool compresses the ROM in memory to reduce the number of times you need to perform the swap, but mntorankusu warns this isn't an exact science at the moment. "On games that don't compress well, the extra overhead actually makes it worse (and slower). I'll try to improve that."
The dev also warns that this approach comes with risks:
"While I've tested this repeatedly and caused no damage to any of my hardware or cartridges, it is not recommended that you use this if you are worried about damaging your console. I reckon that it's particularly risky if your system has been modified to remove the region keyed cartridge tray insert, as its absense may make it easier for the cartridge to be inserted out of alignment, causing cartridge pins to connect to the wrong slot pins or bridge them together, etc. It's also not recommended if you use a bare PCB flash cart or game cartridge, or if your situation has any other factors that might allow the cartridge to be inserted crooked, off-center, etc. Use the software at your own risk! You have been warned!"
The developer is also unsure if people will actually use this tool, but feels it is "quite important for the legitimacy of the flash-cart-using community for everyone to have the ability to dump cartridges."
mntorankusu adds:
"Now you don't need to commit piracy just to dump a copy of a game you already own to play on your flash cart. You can also back up your saves with this, which is perhaps more useful to the average person. I may add a save restore function if anyone cares, but personally I'm happy just to backup my saves and keep them safe on my SD card."