*nix
*nix refers broadly to systems that are similar and possibly descendant from the Unix system and its general setup.
Linux and BSD (and recently the Mac OSX derivative) are probably the most common unix-like systems.
Others include Solaris, HP-UX, IRIX, AIX, Minix, Ultrix, and Xenix, and more recently Mac OS X (being a BSD derivative).
People may sometimes (mean to) refer specifically to POSIX-compliant unices.