Drupal and Joomla are the typical answers. A lot of hostings provide a one click install.
There are also a few good Wordpress plugins to consider as an alternative.
Elgg was not so good when I tried a few years ago.
If you want more a a forum kind of discussion, there are the good old PHPBB and vBulletin. The new entries are Vanilla forums and Discuss hat are pretty good.
I am not sure what you are trying to create and what kind of interaction you would like.
in my humble opinion, today to bootstrap a community and keep it active the best is a Facebook Group:
- You have mass of people there.
- It is easy to invite people that you have already in your network.
- People receive suggestion to join group where their friends are.
- People see the read notification every time they are on Facebook.
If you just need a splash page: http://www.wufoo.com/ and http://launchrock.co/
If it is more event based: https://splashthat.com/