Just to clarify, Heroku and App Engine are essentially just a new breed of hosts, these applications aren't exactly created with Heroku or App Engine, they are using other frameworks that can be used on any host (or at least a lot of other hosts), they are being deployed and hosted there. Tumblr and Wordpress are primarily blogging engines so unless you're just starting a blog they probably won't get you far. What I would suggest looking into are web application frameworks, for example a lot of people who use Heroku are using Ruby on Rails, you don't need to know anything about Heroku to use Ruby on Rails, instead you can run everything locally while developing and learning and only later do you need to worry about pushing up to a host