I'm looking for a simple clipboard programme for capturing multiple items of plain text such as web site URLs and so on to a single text file.
Each time I copy a piece of text to this clipboard it should be added to the end of the list without deleting the existing contents.
You could try the last free version of ClipCache, 1.41, available from e.g. http://www.woundedmoon.org/
legacy Windows software links (other repositories have it as well). It normally saves each clip separately, but once you have it running, right-click on the tray icon and select Append Text Captures. If you want something simpler that does just what you want and no more, there was a 1994-vintage PC Magazine utility by Robert Lausevic called PasteBoard, and an unrelated 2005 one of the same name by Mike Sanders but that one seems to have escaped from my hard disk - ah, found it online: http://fuzzbox.hyper...asteboard/index.html
You may also like to know that Horst Schaeffer's command-line utility ClipText
for transferring text between files and the clipboard also has an append feature.