Best open source CMS software for building a website?

A friend of mine has been raving about Silverstripe recently, has anyone here used it and have any opinions? I'm looking for a flexible open source CMS package that could be used to build a modern site, something that would allow the site to evolve over time as it needed to without having to spend a fortune on developers each time something was added or amended.

Do any of you have any suggestions or could recommend packages that you use and love? Thanks :)

Squarespace seems to be overtaking wordpress for serious users now

Thanks Jack, Squarespace looks interesting but a monthly fee is a bit off-putting. I'd prefer to build a site and for it to cost X all in. Then if I want to change it or add to it down the line that will cost something but not to pay every month.

A friend of mine used Squarespace and loathed it. Very difficult to customise fully, as I recall, the templating system is horribly restrictive. Wordpress still wins it for me. I maintain something like 30 wordpress sites and haven't yet found anything that comes close to it in ease of use / development.

er, it's Wordpress for sure. Squarespace is mickey mouse, SS and ModX are pretty good for the backend interfaces and permissions for larger organisations to set things for approval etc.
Joel - is this for yourself?

Oh to add, we've used all of the above. 3 times out of 4 we use Wordpress - but that's also to do with the nature of the jobs. It really depends what you wanna do with it, who the admins are, global needs blah blah. Happy chat over a pint

Joel - is this for yourself?

Yes fella it is. Will PM you shortly to chat more :)

Drupal

joomla

Liferay?