The maker of the ViDock Gfx ExpressCard adapter solution has just come out with another adapter to connect up to six displays via USB.
https://gizmodo.com/lightning-review-vidock-gfx-display-enhancer-adds-two-5031028
The dongle comes at $130 and can support 1680×1050 or 1600×1200 on both Macs and PCs. It’s compatible with the latest MacBook and MacBook Pros, for those of you who need more displays than the one Mini DisplayPort can hold.
Each adapter supports one display, and you can hook up to six of these to your PC or four up to your Mac for a super crazy panoramic view at your spreadsheets. Compatibility is DVI and VGA, and should be fine with hot plugging in and out. [VillageTronic]
