As the name implies, this is one of the simplest CMS systems around.
I've searched for a simple CMS solution for my customers and couldn't find one, so I decided to write one myself!

There is also a version that has the ecommerce/shopping cart integrated into the CMS. The shopping cart page is dynamically created and runs via javascript and asp.net.  It's called deCMSorder.

deCMS isn't open source, but it's very reasonably priced.
For $2,000 AUD you can have your site up and running in no time at all and this covers the 1st year's web and email hosting too! 
I've also written a dynamic version of this system, eInfoCMS, which is very similar, plus an additional module for a simple shopping cart/ecommerce solution, eOrderCMS.  For $2,500 AUD you can have a complete shopping solution which is dead easy to maintain.

deCMS covers it all:

• add pages
• update content
• add graphics
• one click regeneration of entire web site
• search engine friendly
• so easy, anyone can use it
• WS3 compliant