In the article, Let Google App Engine Support PHP, we know with the help of Quercus, we can run PHP application on Google Applications Engline(GAE), but how about run WordPress on GAE?
Wordpress already runs on Resin with Quercus, but GAE does not support mysql, so the rest job is convert mysql to the Goole App Engine’s subset of JPA, for details, please visit Quercus on Google App Engine.

The first time i knew Piwik is on sourceforge.net, because it is the “This Month’s Project” of July, 2009, and I was attracted by its brief introduce,

Piwik is a downloadable, open source (GPL licensed) web analytics software program. It provides you with detailed reports on your website visitors: the search engines and keywords they used, the language they speak, your popular pages, and a whole lot more. Piwik aims to be an open source alternative to Google Analytics.

and It cost no more 5 minutes to install on my dreamhost hosting.

It has some competitive features,
1. Piwik’s features are built inside plugins: you can add new features and remove the ones you don’t need. If you are a developer, you can easily build your own web analytics plugins!
2. you own your data: because Piwik is installed on your server, the data is stored in your own database and you can get all the statistics using open APIs (publishing the data in many formats: xml, json, php, csv)
3. the user interface is fully customizable: you can drag and drop the widgets you want to display and create a report especially tailored to you!

And you maybe have interesting in 7 Reasons Why Your Commercial Site NEEDS Piwik…,
#1 You Own The Data
#2 Fully Expandable Feature Set Just Like Wordpress
#3 API Developer Access
#4 Real Time Tracking
#5 Total Privacy
#6 It’s Free
#7 It Will Make You More Money

Like it? download the last version, install and enjoy it, and if you have WordPress blog, do not forget it also has Plugin for WordPress, so you can easily integrate piwik with your wordPress blog.

reference,
Home Page
Online Demo

Google App Engine supports Python and Java now, but does not support PHP, but some body offer a hack to Run PHP on the Google App Engine.
He uses Quercus in GAE.
Quercus is Caucho Technology’s fast, open-source, 100% Java implementation of the PHP language (requires JDK 1.5).
For details, please visit,Run PHP on the Google App Engine

Reference,
Run PHP on the Google App Engine
Quercus
A Demo