Apple's Thunderbolt technology is a great option for expanding the capabilities of a Mac system. Since it extends the system's PCI Express bus it allows vastly more options than USB or FireWire alone by giving the option to host controllers for these and other technologies. If you need a FireWire connection and do not have one, then you can attach a controller for it using Thunderbolt instead of having to purchase an expansion card or having the controller built into the computer's motherboard.
This expansion capability has been a very attractive option, especially for laptop users for whom it can serve to dock a Mac system to a variety of external hardware including audio controllers, USB, FireWire, Ethernet, and display adapters among others, all through one connection.
Since Thunderbolt's debut, a number of companies have jumped on the technology, offering high-speed storage options and small adapters, but to date the only true docking solution has been Apple's Thunderbolt Display, which is an impractical option for those who already have existing displays and is also expensive at around $1,000 (primarily because of the display).
To fill this gap, companies like Belkin and Matrox are developing standalone docking devices that will connect a number of I/O technologies to a Mac system using Thunderbolt. Announced earlier this year, these devices were slated to be out by September of this year, but new developments have the release date at least for the Matrox DS1 moved to December.
Matrox's DS1 adapter initially included USB 2.0 ports, a USB 3.0 port, gigabit Ethernet, audio I/O, and a DVI connection, but the company has also added a version with HDMI output.
For the Belkin ExpressDock, the company has taken a different approach and after initially offering USB 2.0 connections, has switched to offering all USB 3.0 ports, FireWire, Audio I/O, e-SATA, and Ethernet along with a Thunderbolt-out connection. This approach offers more connectivity options than the Matrox dock, but requires the use of an adapter to connect to a display. If you already use an adapter for your Thunderbolt-based Mac, this dock might be the proper solution for you.
Both docks are estimated to cost between $250 and $300, though pricing may change when they are finally released.