Yup. Most of the commands are the same, and software (some, not all) that can be compiled on Linux can be compiled on Unix, and vice versa. The main difference is that some Unixes are actually proprietary (like Solaris, AIX, etc.) Some are, however, open source, like Linux. FreeBSD comes to mind.
I'm actually planning to try out FreeBSD on a spare box. Not quite sure how it'll be, but I think I've learned enough with Slackware that I should have very few problems.