Open Source Solutions for Libraries?

rscrawford asks: "I'm a former web developer who's been playing with open source solutions wherever possible for just over a year, and now I'm on that obsessive track toward becoming a librarian. Now, I've seen a lot of library websites and catalog software packages that are MS-based; what open-source solutions are there for libraries, and where have you seen libraries using open-source technologies (like Linux) to solve problems?"