Here's something else that looks pretty cool. BuckyBase is a data-wiki where pages are like free-form records (dictionaries of key / value pairs) that can also be can be shown grouped into tabular form.
It's fairly simple at the moment (another just-launched GAE experiment), but I can't help thinking that in my "enterprisey" day-job I work with nothing but data which is structured like this : records that are less than normalized, sometimes viewed as separate pages, sometimes squashed into grids. Give it the ability to add a few extra-constraints, some kind of blank form definitions; hook it up to Google Visualization and let it embed Gadgets; add some more sophisticated querying ... and you've got the heart of a small-business data-base application : Filemaker-as-a-service.
And if Google can ensure that Application Engine really is fast, powerful and secure enough to host serious applications, then it looks very promising as the new standard substrate for a whole ecology of this kind of wiki-derived tool.