Boss BF-3 Flanger, works very well. Five knobs: one for quantity, another for resonance, depth and rate knobs as you can find on every other flanger pedal. The must, in my opinion, is the fifth option, which is composed of the following options: momentary, standard, gate/pan and ultra mode. It is affordable, light (as the other Boss pedals), very easy and it has a mono/stereo output, which is great if you send it into a reverb/delay pedal.
Also, I second Mr. Wertham about the EHX Flanger Hoax, which is insanely good, but damn huge. I used to do feedback loops with it before sending the signal into a chain of pedals. It can get some unexpected results when it comes to noise. I hadn't the chance to try on vocals though.