[drivers99]

"You could print a line in bold, for instance, by issuing a CR without an LF and then printing the same line again."

Last I checked, this still works even on laser printers (at least on a LaserJet), when sending data to it as plain text. It's not actually printing over itself, but it knows to make the repeated characters bold.

[mzs]

less (among other unix tools) does this too (but you have to do one character, bs and the character again). There are more, like _, bs, character underlines (like cat there is ul that handles this specifically). If your terminal supports os (overstrike) in it's terminal description it handles that natively.