K3B is an application that ships in the default KDE packages in most distributions, however it's a very useful tool that can often be overlooked.
K3B is a CD and DVD burning application that makes it easy to burn data, audio and disc images onto writable CDs and DVDs. It is a very mature, stable and fully featured application.
Open up K3B and you are presented with a split view, with directories listed on the left in a tree structure, the current directory's contents on the right and a welcome page at the bottom.
On selecting a task, such as creating a data CD, the bottom view turns into a view of the files that will be on your CD (data, in this case, but obviously it will be different for audio and other projects). You then drag and drop files from the filesystem into your project as it's called. Once you're done, you can Burn the project.
The whole burning process is very clear and should be easy for anyone familiar burning CDs with software such as Nero over on Windows to pick up and understand.
There are also options for burning images and doing 1-to-1 disc copies available from the Tools menu.
One of the main benefits of K3B is that it is such a mature project. While the tree-like interface might not be to everyone's tastes and it does occasionally seem a little outdated, the application is simple to use and intuitive for people who have used other burning software before elsewhere. It is extremely stable in my experience, and it gets the job done.
For KDE users, K3B is highly recommended for an easy, yet powerful way to burn data, audio and more to disc. GNOME users may want to try Gnomebaker, and if you're into using the command line, give cdrecord a try.