Insert appropriate line-end character (like ';' for C*)

[Josh]

I think I've seen this behavior in Emacs somewhere, but I can't
remember where and I'm either using the wrong terms when searching or
I'm looking in the wrong places because I can't find anything about
it. What I'm looking for is a emacs command that inserts the
appropriate end-of-line character based on mode. For example, if
you're in c-mode or java-mode or whatever it would insert a ';'.

Ideally it would be context aware so that if you were in python-mode
it would add a ':' where appropriate, but not elsewhere (and even a
',' if you're making a list), but that's not essential.

Does this already exist? If not, does anyone have any pointers on how
I could go about writing it? The only part I can't figure out is how
to determine what the right character is.