Developing for Google apps. What do you need?

Image via CrunchBaseJust having a bit of a Google day to day. Started talking to someone about Google Web Toolkit and App engine. From that I realised I hadn't looked into how well it scales and how robust it is. I've just been assuming it will grow in time and do everything :-)
Had a quick look at the Google Marketplace and confirmed that GWT apps could be placed there. The marketplace is available for those using Google Apps for domains where you get google docs, mail, calendar and sites designed specifically for business use.
From there I checked out the options for building for Google Apps. It's nice to see you have a frew options from simple javascript applications right up to copmlex java and python based fully hosted applications. Turns out it's all about apis and I promptly found a link to all google apis on code.google.com
Since I'm becoming a big GWT and appengine  fan I'm sure these links are going to become very useful very soon.