Running out of filament and losing your long print job is a waste of time and filament. Filament anxiety is not fun, and manually tracking filament usage is a hassle.

ToyBuilder Labs sells the Tunell 3D Printing Filament Monitor which adds more sophisticated sensing to a wide variety of printers. Besides detecting running out of filament, the Tunell monitor also detects other filament feeding problems such as the filament getting stuck on the spool, jams cause by nozzle clogs or out-of-tolerance filament, and filament breakage.
When the monitor detects a problem, it sends a signal to the printer to pause the print job so that you can fix the problem. For large and long running prints, or for manufacturing and production environments, this is a crucial feature!

The Tunell monitor is designed to connect directly to the MakerBot Replicator and other clones such as the FlashForge Creator series, and the Wanhao Duplicator 4 series. Other printers work well with the Tunell monitor -- we have had customers using it with Ultimaker's and with various RepRap printers.
The Tunell monitor can be found at ToyBuilder Labs.
http://www.toybuilderlabs.com