The Circlet from KUM does away with the need to use set squares, rulers and compasses. It's a clever, all-in-one instrument that's easy to use and comes in two sizes: 16cm and 22cm. Despite what it's called, it's not a circle, it's a triangle, with a fixing pin in the middle of the long side. Use the fixing pin to fix the Circlet to your paper, but take care as it's fairly sharp! Then use the appropriate guide holes to draw circles and measure angles. Sliding the Circlet along a ruler lets you draw parallel lines, or add very neat cross-hatching.
The video above should give you a better idea of what it can do, but we should say that we found we had to hold the red 'button' down to keep the Circlet in place when drawing circles.