SDF Public Access UNIX
sdf.org
A free shared UNIX system running since 1987 that gives anyone a shell account, email address, web hosting, and access to a community of thousands of technically curious users worldwide.
Why it is useful
One of the oldest continuously running public internet systems, still actively maintained. A free account gives you a real UNIX shell, the ability to host a static website or gopher page, and a community of users who remember what the internet was before the web dominated everything. Useful for learning UNIX in a real multi-user environment and experiencing computing culture that has largely disappeared elsewhere.