A 660AV is something of an odd duck and therefore used to be a bad candidate for running non-MacOS operating systems on. First thing that comes to mind is NetBSD, and NetBSD's machine status document:

http://www.macbsd.com/macbsd/macbsd-docs/machine-status/

Says both ethernet and sound are broken on that machine. Meanwhile, OpenBSD says:

The following Macintosh models are supported and tested. This means that at least the SCSI controller, serial console and on-board ethernet will function on these models. On some of these machines, a full 68040 CPU is required to replace the default 68LC040 CPU.
* Classic family: SE30

* LC family: LC, LC II, LC III, LC III+, LC475, Performa 400-430, Performa 450, Performa 460, Performa 475, Performa 476

* Centris family: 610, 650

* Quadra family: 605, 610, 630, 650, 660AV, 700, 800, 840AV, 900, 950"

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However, OpenBSD also classifies their entire Mac port as "Stagnant":

"The following platforms have been supported in the past, but releases are not being built for them at the moment. However, they are still living in the source tree, and releases might happen again in the future.
luna88k Omron LUNA-88K and LUNA-88K2 workstations

mac68k Motorola 680x0-based Apple Macintosh with MMU"

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The Mac Linux page optimistically says the 660AV works:

http://mac.linux-m68k.org/status/MAC_MODEL_C660.php

The one thing you definitely cannot run is A/UX. The 660AV is also somewhat limited in what MacOS-es it'll run, being confined to 7.1-something to 8.1. (No System 6 or other such historical oddities.)

If you're looking to run weird OSes just for the sake of saying you've run them an old Pentium box would probably be a much better choice.