There are a number of projects people have put together to enhance the classic mac experience. This page tries to provide a central reference for all the known projects that are available.
BMOW's FloppyEmu houses floppy disk images on an SD card, and presents an emulated floppy drive to the Mac. It emulates 400K, 800K, and 1.44MB floppy drives: http://www.bigmessowires.com/macintosh-floppy-emu/
The SCSI2SD project is a microsd SCSI device. Ditch your old, noisy, unreliable SCSI hard disks, and get a SCSI2SD: http://www.codesrc.com/mediawiki/index.php?title=SCSI2SD
Programmable ROM SIMM
mac68k.info's very own dougg3 made a programmable ROM SIMM that works with most any 68k Mac that has a ROM SIMM. This includes the IIx, SE/30, IIfx, IIsi, IIcx, and IIci. Other models may have solder pads for a ROM SIMM socket to be added. Customize your boot chime, happy mac, eliminate the slow RAM check on boot, boot from ROM, and more!
Get a 2MB or 8MB ROM SIMM, and accompanying ROM SIMM programmer: http://www.downtowndougbrown.com/programmable-mac-rom-simms/
PS2 Keyboard & Mouse Adapter
This adapter lets you use a PS2 keyboard and mouse with either an ADB or pre-ADB (128k, 512k, Plus) system: http://www.synack.net/~bbraun/mackbd/index.html
Mac Plus ROM Expander
Get up to 1MB of programmable ROM space with this ROM adapter. Boot directly from ROM! http://www.synack.net/~bbraun/plusrom/index.html
Mac II/IIx Battery Adapter
Another mac68k.info member project, the II/IIx Battery Adapter board lets you replace the soldered batteries on the logic board of your Mac II or IIx with battery holders, similar to most subsequent Macs! This makes it easy to replace or remove your system's batteries, particularly useful for storage, to prevent damaging battery explosions. https://mac68k.info/forums/thread.jspa?threadID=320&tstart=0
Macintosh Portable RAM Card
This card for the Macintosh Portable (both backlit and non-backlit) lets you get up to 9MB of RAM! https://68kmla.org/forums/index.php?/topic/23763-designing-a-portable-ram-card/