yeah, there are a couple of things that kinda fit that bill.
1 is harmonics. and the 5th fret is a perfect example of this. if you lightly touch the string at the 5th fret, you get a harmonic that will ring.
feedback is another, and if youre talking about letting a note ring forever, thats probably what youre asking about. and all you have to do is let it ring and maybe get closer to your amp. make sure you hold that note and dont move the string too much. you can change the pitch by bending the string a little bit. but bend it too much and youll lose it. also, certain frets are better at this than others. so fool around with it.
i often do this for one song for kastigation. i hold and bend the 7th fret. i dont pick it. i just bend it and wait for feedback. and when it comes i SLOWLY let it go. awesome effect.
~gR~