The Roku XD ($69.99) and Roku HD ($79.99) are the step-down versions of the Roku XDS ($99.99).
The Roku XD is mostly identical to the XDS, except that it does not offer component video and optical audio output; it does not include the USB port (for future local media playback); and it offers single-band 802.11n Wi-Fi (not the better dual-band 11n found on the XDS). If you don't need any of those features, you can save $20 by going with the XD.
(Shoppers should note that the Netgear Roku Player NTV250 is just the Roku XD sold under a different brand.)
The Roku HD is the entry-level model in the 2010 Roku line. It's available for $69.99 with free shipping, or $59.99 plus shipping. It's similar to the Roku XD, but it lacks the enhanced remote control; it doesn't have the capability to output 1080p video (just 720p and standard-def); and it has slower 802.11g Wi-Fi instead of 11n.
CNET recommends the Roku XDS as the best choice, with the Roku XD being an acceptable substitute (with a $20 savings).