The $80 Roku 2 XD sits in the middle of the current lineup of Roku streaming media boxes. It's very similar to the Roku 2 XS, but it lacks that model's Ethernet port, USB port, and included motion remote.
All of the Roku models can turn any TV -- an HDTV or an old analog model -- into a "Smart" TV, with the ability to stream hundreds of online video and audio channels and services, including Netflix, Hulu Plus, Amazon Instant Video, HBO Go, Vudu, Crackle, Pandora, Mog, Rdio, and MLB.TV. (Most of the best channels require separate subscription fees, but some -- Crackle, Pandora, and others -- are completely free.) All models offer wireless Wi-Fi streaming and a universal search function (for finding content across multiple video providers). Rokus can also stream photos, music, and video from smartphones and tablets via the free Play On Roku app (which can also double as a remote control), and stream media from PCs and Macs with the Plex app.
But there are some differences among the four models. Here's how the Roku line breaks down:
Roku LT ($50): The entry-level Roku includes all of the features above and retails for just $50. HD video output is limited to 720p, and it includes standard RCA (yellow/red/white) jacks for connecting to older analog TVs.
Roku HD ($60): The Roku HD is basically identical to the LT, except for the color scheme (black instead of purple). Unlike the LT (mostly an online-only product), the HD is available in many brick-and-mortar stores. The list price is $60, but it's sometimes discounted to as low as $40.
Roku 2 XD ($80, reviewed here): The Roku 2 XD is a slightly smaller box than the LT/HD models, and it adds full 1080p HD video output. It supports an optional Bluetooth motion remote, but only a standard infrared remote is included in the box. Analog video output (for pre-HDTVs) is enabled via an included breakout cable.
Roku 2 XS ($100): The top-of-the-line Roku also offers 1080p video output, an Ethernet port, and a USB port for limited local file access. It includes a Bluetooth motion remote and a free copy of Angry Birds.
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?