Thinstation
X2OS
iPXE (you can use this to chain load other thin OS's over the internet, nfs, smb, etc..)
Very small, and configurable. Supports some wireless cards.
ubuntu ltsp! Zero clients that boot over pxe (thinclient or fatclient options)
windows XP, Vista, 7, 8, embedded, CE, etc…
If you already have a windows device, you can convert it into a thinclient using:
thinwin
Commercial options
Wyse ThinOS
These two support SPICE! For VDI deployment to KVM-QEMU
nComputing
VDI Blaster http://www.devonit.com/thin-client-solutions/vdi-blaster