Klipper is a KDE tool which sits in your tray and keeps track of all highlighted text and then allows you to copy this text into documents. It can have a huge history and keep track of litterally thousands of previously highlighted textblocks.
The Klipper version in KDE4 RC2 is totally broken. It is, luckily, possible to use the Klipper version included in KDE 3.x in KDE4.
By 2019 we have KDE5 and Klipper works well.