TikiWiki is very nice in some ways. But it is extremely slow.
Even with the use of the PHP accelerator code like APC or MMCache I found its performance to be very poor.
It would take more than 1 second to generate a page in some cases.
So effectively the server could only serve 60 page hits per minute, max. On the same hardware, an out-of-the-box OpenACS could serve 9 pages per second (540 page hits per minute).
Nice features:
if a module is installed, it can be set to automatically show up as a portlet on the home page
any page can be turned into a PDF by clicking a link
it keeps track of where you have been on the site, so you can easily go back to where you have been
There are others as well. However, I would say that it is more difficult than OACS to customize, due to their using so many different PHP packages (Smarty for templates, ADODB for database access, etc.), which require you to know all about how those work in addition to knowing how everything fits. For instance, Smarty uses C-style date specifiers, while PHP has its own.
