There is a site called GenesReunited http://www.genesreunited.co.uk/default.aspx?ixc=u
, that is popular in the UK.
I don't think it will let other people amend it, but they will be able to view it, if you allow them. Also you can publish it, so that other people that think they share ancestors can email you. Having said that I think you have to pay to use it.
If you save your family tree as a GEDCOM file. Most software can up load them.
In terms of Linux software, I use 'Gramps'.
I think Family Tree Maker can publish ready made web pages, but is windows and you would have to host somewhere, and you would not be able to amend online.
Chrism01 and Chris106 - maybe you are distant relatives.