There are literally hundreds of such applications available.
Try Open Source CMS click here which allows you to 'try before you buy' by using an online testing process where you get a taster of the control panel and output of each CMS.
Xoops is an excellent platform, if not for the easily terrified. To be honest though, you'd have to go a long way to find something more potentially powerful than Mambo. It takes a bit of getting your head around but you really can do some incredible things with it.