The Arduino IDE editor's lack of support for X11's select-paste mechanism has always annoyed me. Well, I finally got around to adding it. Especially for helping people with their Arduino troubles (which I do every day), it's so very nice to finally be able to quickly select-paste between Arduino and forum messages, email, terminal windows, etc.
This is a Unix/X11 feature. Mac & Windows do not have anything similar. But on Linux it's so very fast and convenient. This tiny little feature really makes me happy. :-)
Update: Arduino 1.0.5 now has this feature on Linux. It's not mentioned in the release notes, but if you try select-paste, it now works properly even without installing Teensyduino.