I'd say Dropbox can be fun for quite unusal reason - their software. Custom-built CPython with encrypted .pyc files (some of them are free software, so there's a known-plaintext) and without any useful introspection modules left. Aren't this fun?

Disclaimer: Regrettably, Dropbox TOS forbids reverse-engineering, so I didn't dare to violate them. ;) (Even though local laws here, in Russia permit some limited reverse-engineering for some conditions.)