I have very fond memories of using Opera Mini on my N-Gage and on a bunch of later feature phones that only had J2ME support. It felt revolutionary at the time, especially since data plans were still extremely restrictive.

Wifi on phones was also not quite a thing yet, so I had found an application for S60 phones that allowed them to share a computer's connection via Bluetooth. The range was extremely limited, but enough for me to browse the internet from my bed!