Back in the day, access to the Internet was very expensive in my locality. We only had mobile data with max 20GB per month, and after that - 7KB/s unlimited for ~20% of average local salary. Having no job, it was very hard to pay that amount for the Internet, so I came up with a solution. I have set up a cloud server for 10$/month, got couple of SIM cards with that internet plan, and started developing a Linux tunnel program, which would join bandwidth of multiple interfaces into one by directing packets to my server. The server then forwards the packets further. I hadn't used it for long but it was very fun and satisfying to do it.