[lindig]

The public transport service in Hannover/Germany once had a screensaver that you could configure to show the next departure from your nearest station. I thought that was clever marketing. Today you probably could implement this as a web service.

[FredPret]

Screensavers can / could ping an API?

That train schedule seems like a cool idea in and of itself though.

[simoncion]

In the Windows 95 days (and probably in the Windows 2000 days, and maybe also today) "all" a screensaver was was an .exe renamed to -if memory serves- .scr.

There may have been some special interface that the program being run was expected to conform to so the screensaver subsystem would invoke it, but (IIRC) a screensaver could do anything an ordinary program could do. (That was the big reason for being cautious about where/who you got your screensavers from.)