NoPass type devices are inserted into the DS slot and trick the DS into allowing unsigned DS code to run from the GBA cartridge slot in full DS mode.
NoPass is the third generation of PassMe type devices. The name "NoPass" was coined by the head developer of DSLinux, PepsiMan. They're the same size as an original DS game card. They work completely stand-alone and do not require an original DS game or any kind of firmware programming and work with any type of DS with any firmware, including DS Lite. It is thought that Nintendo cannot make NoPass devices stop working on future firmware revisions without moving to a larger firmware chip like that used in iQue DS.
NoPass contains DS code that's encrypted in the same way original DS games are encrypted. The DS recognises and runs the NoPass as any other DS game. This means that unlike the earlier PassMe devices NoPass does not rely on security holes in the firmware or bios to get into full DS mode.
- EZPASS-3 from the EZ-Flash company. (sleep supported)
- Action Replay DS from Datel, version 1.02 or later, hold select as you're booting up. 
- MAX Media Launcher from Datel, also sold as part of the MAX Media Player package (sleep supported in homebrew only for first ones, later revisions should work with commercial games as well)
- MK4-Key from the company behind NeoFlash, barebones NoPass. (sleep supported)
- MK4-mini from the company behind NeoFlash, with some extra features. (sleep supported)
- Ninjapass Media Launcher from the company behind Ninjapass DS Flash.
- PassCard 3 from the company behind M3 and G6. (sleep supported)
- SuperKey from the creators of the SuperPass and SuperCard product lines. (sleep supported)
PassCard 3 and SuperKey are reportedly identical in all but the label on the card. 
R4 and most other SLOT-1 flash cards can act as a NoPass, either through a button combination, through a menu option, through a special .nds file, or simply by leaving the memory card slot empty (as on Games n' Music).
M3, Supercard and G6 are all confirmed as working with MAX Media Launcher, PassCard 3 and SuperKey.
EZ-Flash IV does not work with certain versions of PassCard 3 and SuperKey. The only NoPass device confirmed to work with it is MAX Media Launcher. 
Overall it seems MAX Media Launcher is compatible with most (all?) flash card kits, while PassCard 3 and SuperKey has some compatibility issues with certain cards.