I already bought the Nokia N900 and it's seems to be a very good cellphone but the main objective in to use this cellphone is to have possibility to use another operating systems like Android 2.2 and i want to run the Meego Comminity Edition and i would like to know if this operating system is useful enough and i want to know too if we can run softwares build for Nokia N9 that runs the Meego Hartmann in community edition version.

short answer no

long answer: well its not longer meego community edition but now known as Nemo Mer, and yes you can run some applications from Meego Harmattan but, it is in no way an everyday operating system or ready to be that just yet, more work is needed, same with Android 2.3.4 it still has no voice calling while sms works fine but the battery drains like a mother ****er.

Well, i think that is possible to believe that Maemo CE can be a very good alternative operating system and if Maemo CE is compatible with some softwares build for Maemo Hartmann, this can be a new life for N900.

Someone expects that Maemo Hartmann will be a good amount of softwares or this operating system is only a test operating system for geeks users?

MeeGoCE has been replaced with MER/Nemo(GUI) or MER/Plasma etc.
Mostly the same underlying stuff. For a background why etc, ask in the MER IRC channel.

Harmattan is the first Maemo that is marketed actively and available from operators in some countries. As such there is already a fair amount of software available and it's growing more every day.
Nemo can indeed run some Harmattan apps. But you cannot directly install Harmattan apps on Nemo. You would have to manually copy the files over.
There is an article on meegoexperts.com about running Harmattan stuff on Nemo on the N900.

The Maemo Community Edition Wrecks The Whole System,Allthogh The Potrait Oriention,The Omp And Other Features Are Allright..But The Speed Of The Fone Is Pathetic..Music Player Automatically Stops
Camera,after taking 3 4 shots turns black..And If The Fone Hangs Out..You Have No Other Opt Other Than A cold Restart

Giving the recent developments with stable framerates + higher resolutions for video recordings, i do not see a necessity to substitute Maemo 5 + CSSU with something different other than for developers & HW hackers. I freshly installed mine yesterday and it's a breeze.

If you want Android, buy a cheap Android cellphone with unlocked bootloader to use it with CM. If you want something with a "Meego" base, either buy an N9 or... dunno.

Maemo is perfectly fine.

If you want something with recent / popular "apps" (like that Whatsapp crap), buy a cheap *droid anyway. I still have to read valid reasons why people want to taint a N900 with Android or other non-finished stuff, when all they want to have they could achieve with a cheap (possible chinese) *droid.

Apologies for the blatent culling.
Yes, in manyways you're right. the CSSU and other OSS that makes full use of the available hardware, massively improves matters.

In many ways, there is still no device like it. No true N900 user wants to lose it. The area that bugs most users is, the dichotomy of the always-online/desktop-in-your-hand philosophy against the way some sites treat N900 users like lesser individuals; for example, browser detection (workaround available) and the flash 10 problem. I suppose these are the reasons one might wish to use another O/S (other than proof of concept).

With browserd now open, there is a greater opportunity to improve the inner workings of microB to HTML5, which might overcome the problem of embedded videos?

@OP: Mobiles & PDAs are an embedded devices, the N900 was designed with/for Maemo5 so being able to use another O/S is a happy coincidence of it being near-full Linux, not the intention of device design. In short, if you prefer another O/S, you'll need to buy a device that includes it.