Sleep mode and months of operation on small battery In context of getting the MCU on Pro Mini and ATMega2560 (chip is ATmega328 and ATMega 2560) to sleep and power down modes, so as to run on one set of battery for months, appreciate idea and comment.

Initially, I looked at web postings and get the following general idea. Did I miss anything?

Is sleep mode done via ATMel factory library, Arduino-level user contributed library, or the Arduino IDE? Many web posting are along the line of getting it partially working but only at half expected current reduction, etc. That is, current do reduced, but, not enough. 

Are there several such library? 

How are they? 

Which one is better support and more functions? (no need for easy to use, can do registers programming, when needed)

Many thanks in advance
Have a read through the Jeelabs blog which documents the author's very successful attempt at making a very low power wireless sensor node that can run for months off coin cells.

http://jeelabs.org

There is also a library based on this project called Jeelib:

http://jeelabs.net/pub/docs/jeelib/

Jeelib contains a class called Sleepy that allows for easy sleeping.

Rocketscream did a thorough test of the current consumptions with the different modes here:

http://www.rocketscream.com/blog/2011/07/04/lightweight-low-power-arduino-library/

According to them, you can get the ATMega328P down to 1.7uA.

Unless you do something about the power supply on a normal Arduino board you won't be able to extend the battery life by much at all. See my answer here for some more details:

Arduino powering from 9V battery