Accelerators

There are lots of system 6 compatible accelerator cards around. The Daystar powercache and turbo come to mind, but the sonnet presto 040 is probably compatible too, so are the micromac cards and numerous others.
You'll find some drivers on this page

Computers

Macintosh 512ke
Macintosh plus
Macintosh SE
Macintosh SE/30
Macintosh Classic
Macintosh Classic II
Macintosh Performa 200
Macintosh Portable
Macintosh Powerbook 100
Macintosh II
Macintosh IIx
Macintosh IIsi
Macintosh IIcx
Macintosh IIci
Macintosh IIfx
Macintosh LC
Macintosh LC II

Disk and CD Rom Drives

A few system 6 compatible software drivers are available here

Ethernet cards

You can download system 6 compatible drivers here

Miscelaneous Nubus cards

You can find drivers on this page

Printers
(this list is definitly not complete)

All postscript laserprinters that can talk appletalk. Read all about Apple Laserwriter driver compatibility here.
Image Writer I and II
Star SJ-144 MC
StyleWriter I and II
HP Deskwriter (all (color) inkjetprinters supported by the hp deskwriter 6.0 driver software): C, 520, 540, 550c, 560c.
You can find drivers on this page
With the help of a Unix or OS X print server it is also possible to print to much newer or officially incompatible printers. You can read here how it is done.

Scanners

Among the scanners that are system 6 compatible are the HP scanjet II and the Apple Scanner One.
Drivers for older scanners are to be found on this page

Video cards

You will find some system compatible drivers here