Pseudo Disk ][ Controller Card
Nice project. The megaAVR I can handle, the Xilinx CPLD puts this over my head.
SDISK II - DISK II emulator for APPLE II (or version 1)
Cool, and accessible. The device uses a megaAVR, but uses the the standard Disk ][ drive controller card. This is great because it potentially allows an Apple //c without slots to get in on the fun using the built in drive controller (but no slots). It appears to use the same, or similar, megaAVR code as the Pseudo Disk ][ Controller, above. This design appears to have a hard time keeping up with the bandwidth required based on the over-clocking that is necessary. I don't like the use of non-standard disk images either.
Semi-Virtual Diskette (SVD)
Uses SRAM instead of SD card. Requires special PC only software.
Nice looking product. Compact Flash. Requires a slot. Not DIY.
Smartport Virtual Hard Drive
This looks very doable. Unfortunately, I don't have a SmartPort based computer! My Apple //c only has the original version 255 ROMS. I like the use of commodity hardware, but the Micropendous board runs about $60. Certainly affordable, but I am trying for cheaper.
HxC Floppy emulators
Very nice! - if you have any type of computer except an Apple.
Macintosh Floppy Emu
Whoa! Very cool. Maybe one day... Macintosh only. not DIY.
So, I am going for something like the SDISK II, but I want to use strictly SPI to make the transfers to the Disk ][ controller card. Based on my reads of "Understanding the Apple II" (82Mb PDF book download), I think I can multiplex all of the Disk ][ signals into an SPI standard stream. This should free up some processing power on a micro-controller with native SPI, and should free up some pins for an interface to select the active disk image.