There are many bridging calibration test pieces out there and I was inspired by the thingiverse hollow cube and quickly drew up my own 30mm cube in Sketchup and printed it as my calibration piece. It makes a nice 'give-away' printed object!
Technical note: I printed it at .3mm height, .45mm width, slow speed. I'm currently using Slic3r 0.9.1 from within Repetier-Host (v0.70b). I'm happier specifying both height and width in Slic3r at the moment for what it's worth, following Nopheads guideline 1.5 ratio (.3mm *1.5 = .45mm).
The bridging works best for me when I set a Bridge Flow Ratio of .9 (in Slic3r... Print Settings... Advanced). That supplies less plastic than would normally be laid down for the given distance, therefore stretching the plastic across the gap. As the plastic cools it also tightens up, pulling straight. This can be helped by a fan blowing cold air at this point of the print.
I'm using 3mm PLA with a .5mm nozzle, printing at 185C (195C first layer). The only cooling going on is any air drifting to the work from the x-carriage mounted fan that cools the hot-end insulator (peek part of the j-head).
I captured the bridging moment on video for people to see bridging in action! Also visible in the opening minute is my new y-carriage, PCB heated bed and the ribbon cable connected beneath it, for power and thermistor. I thought that might be of general interest also.
Thanks for viewing!