Opensource Alternative to Share point

Hello all,
I'm almost being compelled to implement Microsoft Share point Server.
However I have some reservations cause I'm traditionally a PHP/MYSQL LINUX Developer.
Please If you've used Microsoft SharePoint before;
what exactly are the features you used ?
Is there an open source alternative you've seen heard of or used before ?
Your response will be appreciated.

Please make this popular by digging so more people can see it and help if possible.

Thanks.
Prosper.

A blank Slate

Millions out of the millions that visit www.google.com each second fail to realise how therapeutic that page is.
I recently rid my desk of all projects. No clients calling to ask questions or make request. No one making suggestions. Just me a blank desk and a free mind. It feels like a breath of fresh air. Don't get me wrong. I had my finances in order to last a while.
But the feeling is just so strong and healing. 

The thought of doing exciting, that will have zero compulsory demand on my mental, physical, or spiritual resources. The first thing that came to mind was to visit google.com and once I got there, I felt seduced by the clearness and blankness of the page that I stayed there for a few minutes just savouring the power it gives me.

The power to choose where I want go, what I want to see, hear, or feel without any chiding or influence from any corner.

So much power. 

Yet it goes unappreciated several million times a minute.

I feel like a google home page.

So powerful.