Note-taking applications are far from scarce on GNU/Linux desktops. If your needs are simple, you can use KNotes in KDE or Sticky Notes in GNOME. If you want integration with address books and email, you may prefer Evolution's built-in Memos pane. For those who need more than basic notes, the increasingly sophisticated Tomboy may be a solution, assuming they have no objection to running an application built using Mono.
However, by far the most versatile note-taking application is KDE's Basket, a tool so flexible and complete that you might prefer to think of it as a personal wiki, a producer of scrapbooks, or even a creator of temporary desktops. The future of Basket as a project is uncertain at the moment, but that doesn't mean that you can't take advantage of its power.