The HiTechnic IRLink is two-way IR communications sensor. This sensor can be used to communicate with a LEGO RCX or to control LEGO Power Functions motors via a LEGO Power Functions receiver. With the new IRLink IRBeacon block it can also be used to create an infrared beacon signal for the IRSeekerV2. This block lets you choose between sending a 600Hz or a 1200Hz signal which can then be detected by an IRSeekerV2 sensor to determine the direction to the IR source. You can use it on one robot to have another robot follow it. This block can be used by itself as a one block program or it can be used in a loop and you can control when you want the block to be active or not.
Update (Nov 2011): Now also available as a LabVIEW 2009 VI. This VI should also work with LV2010/LabVIEW for LEGO Mindstorms