The U.S. Air Force is equipping a hundred of its F-15Cs with heat sensing pods, that used to equip navy F-14Ds (which were retired over the last few years). The refurbished navy IRST (InfraRed, Search and Track) pods enable the F-15s to detect and track aircraft, over a hundred kilometers away, from the heat the target aircraft give off. IRST is a passive (it does not broadcast) sensor, thus it is undetectable by the enemy.
IRST has its limitations. The main ones are range (usually about 30 kilometers for accurate detection) and problems with clouds and have distorting the heat signature of the target. The short range means that another aircraft using its radar (which has a range of over 100 kilometers for precise identification) has an obvious edge. The distortion problems are slowly being solved by improved computer analysis of the detect image. Since many warplanes like to operate "quiet" (without any electronic transmissions), IRST becomes the best way to spot the other guy, and open fire, first. IRST is also capable to spotting stealth aircraft, which are protected from radar transmissions, but still have jet engines throwing off lots of heat. The F-14D IRST was notable for its long range. But at more than 30 kilometers, the IRST gave pretty vague data. Still, it's believed that just having an indication that someone is out there, more than a hundred kilometers away, gives you an edge.
The U.S. Navy is also installing a new IRST in its F-18Es, which will be mounted in a modified centerline drop tank (which will contain the IRST as well as 68 percent of the usual fuel). One problem with this approach is that the F-18E can't jettison this drop tank, to make itself more maneuverable for air-to-air combat. Other aircraft, like the F-22 and F-35, have the IRST built into the fuselage.