For files? An unholy amalgam of snake, kebab and dot, depending on what exactly I'm trying to convey. (I still have much-ported files around with DOS 8.3 filenames, so they're truncated scream case (SCREAMIN.NAM), but I don't actively name files like that anymore.) The important thing is to separate the words/sections while using characters that are valid without escapes in both ext4 and vfat if at all possible.

For variable names, camel or Pascal case (depending on language convention) if I think anyone else is going to read it. Flat case for code I don't think anyone else is ever going to see (don't do this—it has left teethmarks in my ass from time to time and will do the same to you).