I just bought a Coral M.2 Accelerator A+E key after seeing a lot of buzz about this little 'IoT' TensorFlow-compatible AI accelerator.
I also just received an M.2 A key to PCIe 1x slot adapter card (see https://github.com/geerlingguy/raspberry-pi-pcie-devices/issues/38), and I will pop the Coral board into there.
I haven't done much with AI/ML, but apparently one of the big holdups for using this board with the Pi may be overcome soon—see: https://github.com/google-coral/edgetpu/issues/280
(And I don't want to step on @timonsku's toes here either—he was the original inspiration for me getting this particular card, after seeing his Piunora; just figured now is as good a time as ever to take a quick stab at TensorFlow.)
See related, in the Pi Forums: https://www.raspberrypi.org/forums/viewtopic.php?p=1772610&sid=4833ac3f714618282207affca2bcd846#p1772610
And the patch advertising MSI-X support in the Pi Kernel (currently only on 5.10.y branch): https://github.com/raspberrypi/linux/commit/6bf63f7711b550de8c803a4c4ad792ecfbe721df
(Note that it may be incorporated into Ubuntu for Pi too... https://twitter.com/m_wimpress/status/1345077692568367105