Three handy Needle Gauges from the wonderful ChiaoGoo.
There's the 5" Gauge that measures needle sizes from 1.5mm to 10mm (000 to 15 US)
The 5" Swatch Gauge which not only measures Hook / Needle sizes but it is also a swatch gauge to ensure you get the perfect tension for your project.
Finally the 3" Swatch Gauge which is the 5" Swatch Gauge little brother.