Roku Streaming Stick ($100): This is a "Roku box on a stick." It offers the same basic functionality as the XS (including the motion remote), but it adds dual-band 802.11n Wi-Fi. Note, however, that the Streaming Stick is compatible only with TVs that offer an MHL port, which is still comparatively rare.
Which Roku is best for you?
CNET enthusiastically recommends the entry-level Roku models, which deliver the core feature set for as little as $50 (sometimes less, with discounts). The Roku LT and Roku HD can be considered identical and interchangeable -- just get whichever one is cheaper.
If you absolutely need 1080p video, you can pay extra for the Roku XD -- but be aware that the resolution on most streaming video services won't be sharp enough to see the difference between 720p and 1080p video resolution.
The Roku 2 XS will be overkill for most buyers, but it's a good option if you need the dependability of a wired Ethernet connection.
For more details on the Roku -- including how it compares to the Apple TV -- read the CNET review of the Roku LT.
For a broader discussion of Smart TV options, read Which streaming-media device is right for you?