[nn2]

Variants of large scale Passive Radar http://en.wikipedia.org/wiki/Passive_radar was supposed to beat first generation stealth. I suppose it's a closely guarded secret if it still does for the more modern systems. This also disrupts the standard tactics of taking out the radars first to let non stealthy planes follow.

[runarb]

There are also some research around high frequency radar and its ability to overcome stealth.

The SA-3/S-125 Neva soviet surface-to-air missile system are rumored to have something that resembles such a radar. The Yugoslavia army used one to shoot down an Americana first generation stealth fighter in 1999[1]. It may have been the radar, but there are also reports that the bomb-bay doors was opened, raising its radar signature.

High frequency radar are also very vulnerable to clodering. The version that is known to exist can only be used if the plane is flying above the missile battery. If the radar is illuminating the plane in such a way that there is a mountain or other terrain behind it, there will be to much noise to make out the plane. But breakthrough in digital computer based system is expected to change this in the future.

1: http://en.wikipedia.org/wiki/F-117_Nighthawk#Combat_loss