Re: Cool apps
Truly depends on your interests. As stated, the App Store has plenty of options. However as you pointed out that you are looking for non mainstream apps, again it depends on your interests. Google searches always provide me with options for a specific type of app I'm looking for. For example, I needed an app that would help me organize my ebooks beyond what iTunes offered. I found Calibre. It alone hosts a web server that allows me to download books onto my iPad without needing to sync with iTunes.
Again, it really depends on your interests. I don't play games icon my Mac, only iPad so I can't give you any suggestions.