I/O- Video Interfaces, Capture Cards, Break-out Boxes, Hardware Transcoding

AJA: http://www.aja.com/

AJA makes capture cards for Final Cut Studio (and other applications). AJA also makes adapters and converters and break-out boxes. For the Mac, the capture card "family" of cards is the Kona cards. The Kona 3 allows the capture and output of any SD or HD format, can cross-convert, up-convert, down-convert and can handle dual-link HD.
   

Blackmagic Design: http://www.blackmagic-design.com/

Blackmagic Design makes capture cards and break-out boxes for Final Cut Studio (and other applications). The Decklink series of cards allow for the capture and output of various SD and HD formats. Products like the Intensity allow for HD editing via HDMI input and output. HDLink allows for HD monitoring via computer LCD displays.
   

Matrox (Matrox MXO): http://www.matrox.com/video/products/mxo/home.cfm

The Matrox MXO allows for SD and HD monitoring to an LCD computer display as well as can be switchable to extend your computer workspace when not video monitoring.

Telestream: http://www.telestream.net/products/products.htm

Multiple products for transcoding various formats of video. Includes the Flip4Mac product line for integrating Windows media on the Mac, in Final Cut Studio, and outputting to Windows media formats.