Some handy applications you might not know:

• Easystroke You can assign system-wide mouse gestures to commands and keyboard shortcuts. I use it for a lot of interface functions: close program, close tab. minimize, maximise, go back, go forward, go up, refresh, etc. So efficient, when you get on computers that don't have this, you suffer.
• Guake: a "drop-down terminal". Assign a keyboard shortcut ([Shift][Alt][S] in my case) to toggling the terminal's visibility. Instantly access your terminal all the time, dont loose your windows, etc! (on KDE try yakuake!)
• There's a lot more stuff you can customize that's not in the numerous "System settings" sub-applications. install dconf-editor and playaround in it. Make sure you checkout the org.gnome directory, but browser around because nice tweaks are hidden here and there
• For a good note-tacking application or general content / tasks organizer, try Zim wiki. I've tried several note-taking applications before settling on this one, I use it professionnaly at work to keep organized. One of my best productivity assets.
• Don't like evince's limited interface? For a more decent PDF viewer, try okular. It doesn't require a lot of KDE packages. If you want the best of the best, try running PDF-Xchange Viewer in Wine.
• Disapointed of transmission's primitivity? Miss Window's µtorrent? Try Deluge! Very efficient yet simple GTK+ bit torrent client with a neat interface.
• For a better calculator, try speedcrunch. It has a multi-line output that keeps a history of your operations.