There has been tremendous progress on the distributed control of mobile autonomous multi-agents, mainly driven by the need to achieve desirable group behavior for the multi-agents based only on local information exchange and feedback control. In this review article, a small-gain approach is proposed to design a new class of distributed nonlinear controllers for leader-following formation control of unicycle robots without global position measurements. With the proposed distributed controllers, the formation control objective can be achieved without assuming any tree sensing structures. More interestingly, the distributed controller is robust to position measurement errors and the linear velocities of the robots can be restricted to specific bounded ranges. The generality and the power of the proposed small-gain approach are supported by its application to tackle the distributed nonlinear control of other important classes of multi-agent systems modeled either by double-integrators or the well-known output-feedback form.