Is  there a company currently offering a service which allows
individuals to register with the company and recieve a unique phone
number (or common number+ unique extension) and s/he can distribute
that number to their friends/family/clients so that who ever who wants
to leave a voice mail to the customer can call that phone number and
leave a voicemail, and the customer can pick it up from his or her
email inbox? if yes please list those sites.
Second part is there any asterisK extension to do this easily? i mean
can u point me to any site which talks about this
third part is can you point me to a site which talks about installing
asterisk in an ubuntu system?

Lingo does this automatically with their VOIP services.

http://www.aimphoneline.com/
www.privatephone.com

also www.k7.net

www.j2.com

I accept mister4u-ga comment as answer to the first part of the question.

asterisk includes the functionality you mentioned.  you can setup the
voicemail on an extension to send an email with a wav or mp3
attachment to whatever email address you specify.  i would suggest
taking a look at trixbox which is a packaged asterisk
solution(www.trixbox.com). it is built on centos which is a repackaged
version of redhat enterprise server.

if you want to go with ubuntu, you can use apt-get to install it. you
can also use apt-get to install freepbx or destar which are web
frontends to asterisk.

resources:

http://www.asterisktutorials.com/
www.asteriskguru.com
http://www.amazon.com/dp/1904811930?tag=voipspeak-20&camp=14573&creative=327641&linkCode=as1&creativeASIN=1904811930&adid=02NZKKX2E2FDAWMY6SNP&

http://www.teliax.com/ is the best service provider for this.