One of the killer features of OS X (since Tiger, I believe) is when you right-click any word in a cocoa app, you get a menu with a "Look Up in Dictionary" option to bring up a popup showing a quick dictionary definition of the word and its synonyms. Well, somebody got the bright idea to develop something similar for Linux and it's called Artha. It's in the Debian and Ubuntu repositories, and when you install it, it works in a similar way. You highlight a word and press the hotkey combination (default is ctl + alt + w) and it brings up a window with a definition of the word and also options for synonyms, antonyms, derivatives, and more. In addition to those extra choices, it also outdoes OS X by working within Firefox/Iceweasel, something OS X's dictionary can't.
Launching Artha initially has it running as a background process where you need to invoke it with the hotkey combo for it to do anything. The exception is on first run when it shows you a brief info message and takes you to the main window. Notice the red quit button in the toolbar. If you quit with that, Artha will no longer run in the background and invoking it with the hotkey won't work, so to close the window you'll want to use the close window button only. Also, to have it running automatically after startup, you need to add the "artha" command to whatever autostart scheme you're using.
Artha performs well except for one major bug. When looking up some verbs ending in ing, it crashes with a segmentation fault. Specifically verbs ending in ie and replaced with ing like tying, vying, and lying. Also verbs with a second consonant added like swimming, sitting, and falling. And, inexplicably, matter, but not tatter or slammer. These faults are somewhat predictable and can be avoided, but I'm going to file a bug report against the Debian Wheezy package, anyway. If you can reproduce this bug on Ubuntu, feel free to file a bug report there, too.
I found a couple of other minor bugs. First, there's supposed to be a notification feature where if you hit the hotkey combo it brings up a notification popup with a simple definition instead of bringing up the main window. However, the dependency libnotify1 has been pulled from Debian Testing (temporarily?), so that feature can't work. Also, I tried changing the hotkey combo, but either I did it wrong or it's another bug, 'cause I couldn't change it.
I'll update this space with any news on the ing bug, but I wanted to post about it now because this package shows a lot of promise and is still somewhat early in its development.