UPDATE: I suggest using Arc_Module_2.scad as it's an improved version of the original.

The arc() module is a tool for creating 2D arcs in openSCAD.

The biggest difference is that this module creates the arc from scratch by calculating the vectors of each point, this means the radius at the open ends of the arc is always true and that all the fragments are of equal size.

The ffn (fixed fragments number) parameter gives the option to set a fixed number of fragments in the arc. By default, the number of fragments is set by $fn or $fa, this value
will determine the number of fragments in a full circle. The number of fragments in the arc is proportional to the size of its angle.

Parameters:
a = angle of arc
r1 = radius 1,
r2 = radius 2
t = (turn) rotates the arc like the rotate() modifier
ffn = (fixed fragments number) set a fixed number of fragments in arc
rfn = (round fragments number) round the number of fragments up or down to the
nearest integer

See the notes in the scad file for a more detailed explanation of all the parameters.