... A few favourites:
Dove MacSnap RAM/SCSI upgrade for the 128K/512K -- there were loads of RAM/SCSI upgrade makers, but this was one of the best.
Orange PC cards. Great fun to play with because you have to refresh both your old Mac and old DOS skills. Note: I'm looking for the Windows 95/98 and NT driver packages for the Pentium PC cards.
Mac Recorder -- sound input for 68K Macs. Interesting to use for about five minutes unless you have a project that requires it.
Accelerators that use the IIci cache slot (or via an adapter). Thank you to DayStar and others for delivering some storming fast 68K Macs.