(a) what is your favourite Photoshop plug in ...any why
(b) what is your most used photoshop plug in (as they might not be the same)
It has some stupid tacky stuff thrown into it but there are also some useful glass filter simulations
http://www.niksoftware.com
Tiffen DFX
I always used Tiffen glass filters in the past so I love this one. Has some great filters for brushing skin tones.
http://www.tiffen.com/products.html?tablename=dfx
I hate programs like noise Ninja and any other noise reduction software. They are way overused by photographers. The pictures end up looking worse than they did in the first place. You can spot noise reduction from a mile away .. best trick >> expose the frame so that there is no noise. Then there's no reason to worry about it. :)