I routinely use g, s, and sometimes even i, e, x, c. I even know what o and m means, but I haven't find any use for them.

However, I am all confused when I write regexen in ruby, because the m option for ruby regexps is the same as the s in perl. Even worse, if you write s by habit, you don't even get a warning because it has a different meaning.

Update 2007 jan 22: since then, I've found a use for m as well.