Kensington today announced the sd200V Notebook Docking Station, which is apparently the first Mac compatible DisplayLink adapter. For a while now Windows users have been able to add a secondary display to their system via a USB device. This is the first time Mac users have been so lucky. The sd200V supports VGA monitors with a resolution up to 1440x1050 and also functions as a USB hub with audio, allowing you to connect a monitor, keyboard, mouse, and speakers to your Mac with a single USB cable.
They will also be offering a display-only USB DisplayLink solution called the Dual Monitor Adapter later in the year with a DVI port. Although targeted at notebook users, I think this is particularly exciting for the Mac mini which gains genuine multi-display support for the first time (the Matrox DualHead not withstanding).
The sd200V Docking Station is available now for $139.99, while the Dual Monitor Adapter with ship in April for $99.99. Both devices will be Intel Mac only.