These modestly priced Legacy clip-on tribars from Profile are ideal if you fancy having a bash at time-trialling but don't want to lash out for a dedicated bike. They fit either regular or oversized handlebars, which is convenient. Best not to try them on carbon bars though.
Oval have developed a wide ranging system that offers unparalleled levels of flexibility while maintaining the integrity of a normal drop bar set up. The SCCS system offers road cyclists the option of adding aero bars without ruining the useful centre section of a standard bar set up.