I have said it once and I will say it again give this application the ability to hack out an export for MS Publisher files, and many people could dump Windows forever. That said, Scribus is always under constant development so that the end user has access to the best publishing software possible.
Why Scribus? It's powerful, functional and can anything you would find with closed source alternatives, in my opinion. As more and more people become fed up with the consistent backwards compatibility headaches presented with MS Publisher and other similar closed source products, Scribus will grow. Its ever-growing popularity simply stems from its fantastic abilities and outstanding pricing structure free.
Generally seen as the Linux alternative to Visio, Dia is something of a lesser known application from within the drawing community. But lately, there has been evidence that this is changing and quickly at that.
I believe its value stems from its use over other drawing applications from within the open source world. Addtionally, since it targets Visio right away, it is not left trying to attract new users like Open Office Draw.
Some of you might be surprised that I did not say Cinelerra instead, but in reality, I have found LIVES to be better supported for download, easier to use and more popular overall. The application itself is much more than a mere video editor; it allows the user to go so far as to act as their own video DJ.
Another attraction for some of the advanced users out there is the ability to actually create scripts that allow for even more functionality. It offers real time performance in viable, popular, non-linear editing.
This final application is one that I could have never lived without when I made the switch to Linux Akregator. Unlike Liferea, Akregator doesn't fall on its face when too many RSS feeds have been added to the database. It's rock solid and perceived by many insiders as a bit of a sleeper app, since most users outside of the Linux world are only now becoming more familiar with it.
Much like RSS syndication itself, Akregator is an application that provides the level of RSS browsing functionality the user has come to expect, but doing so in such a way as to not confuse new users with too many confusing features.