The following are copy from several different user's post.
Hopefully this can help more newcomers (or not) to know more about the great Green Button.
Thanks to Peter Zack, RuiC, sezme.

In manual mode (A,FA, DA etc series lens) you can push the button and it sets the exposure for the program mode and the program line. Program line is the setting you have selected in the camera menus, like MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer). Then you can fine tune the exposure to your liking from this setting. Very old school but an absolute requirement for a serious shooter in tough lighting and many other situations. (Peter Zack)

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading. For those that own these lenses, it's an approximate reading and you have to understand (with practice) the way your particular lens will meter on the attached body. In most cases, these lenses meter well wide open but tend to over expose in middle apertures. So you use the green setting recommendations as a starting place to adjust to the correct exposure. (Peter Zack)

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again. (RuiC)
image from Digiichi - Next generation hyperprogram.

From Full Manual mode to P, Av or Tv
The green button in M mode get you to P, Av or Tv dependant on what mode is chosen in the green button menu set up. (excellent if you want to use it like an on-demand spot-meter)

The green button doesn't change from M (manual) mode to any other mode - it leaves you in M mode. It just momentarily gives you the effect of being in one of those other modes (which one depends on how you've set it up in your custom options) - setting the same aperture and/or shutter speed that you'd get it you were in one of those other modes, but then leaving that set. As opposed to constantly readjusting as the light changes.

(thanks lol101, Marc Sabatella, adwb)

Act as Reset button: Reset Exposure Compensation, Flash Compensation
If you have set exposure compensation, hold the exposure compensation button plus the green button will reset the meter to zero E.C.
* You can also reset the Flash Compensation with green button. (thanks rockmaster1964)

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

Zoom in in Playback mode
In playback, after you zoom, Green button zooms IN and EV+- zooms OUT

[Canada_Rockies]

Well written, vizjerei!

[Elgatogrande]

And what about the different uses of the infamous green button on the K200D? Im just getting started and havent figured it out yet.

[tybeck]

Wonderful summary. Thank you. Here are some other links provided in another thread..

Digiichi - Next generation shutter and aperture priority
Digiichi - Next generation hyperprogram

[MacGirl]

Thank you all for this wonderful information. My K20D will arrive Friday so I'm sucking all this up like a sponge!

[Marc Langille]

Excellent summary and follow up!

Regards,
Marc

[rockmaster1964]

The green button will also reset the flash compensation to zero in the flash menu.

[vizjerei]

I just put the information together, thanks to the great Pentax community share their knowledge, as I think Green Button is unique, only Pentax has it, we should not let it just sit there on our camera

And what about the different uses of the infamous green button on the K200D? Im just getting started and havent figured it out yet.

I think it acts the same as in K10/K20. Try it out

Groovy, thanks for that. Didn't know you could quick change the ISO like that.

[lol101]

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!

Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again.

And let's not forget the use of the green button in M mode to get you to P, Av or Tv ponctually (excellent if you want to use it like an on-demand spot-meter).

[kibipod]

great post. thanks a lot.

[bogiesbad]

A wonderful button...and green to boot. Thanks for making this a sticky.

[sebberry]

I'm still confuzzled.. all hope is lost

[grun2000]

And what about the different uses of the infamous green button on the K200D? Im just getting started and havent figured it out yet.

For the K200d:
In playback mode after you zoom, the green button doesn't zoom, rather the AV+/- button zooms in and the AE-L button zooms out.

As a reset, green button also resets white balance compensation in the white balance adjustment mode.

[Peter Zack]

I'm still confuzzled.. all hope is lost

Care to expand that? Maybe we can offer some advice.

Excellent post vizjerei