Normally I wouldn't create a tip to tout a bit of software, but this one is too good to pass up. It's a tiny clipboard history manager named ClipX. Instead of alt-tabbing back and forth between windows copying bits and pasting them you can copy everything you need and then alt-tab over and ctrl+shift+v and hit the number of the entry you want to paste.

Link: http://www.bluemars.org/clipx/

You can use the xclip utility to manipulate the X clipboard from the commandline or from a script. This command will overwrite the current X clipboard selection with the contents of /etc/passwd so a subsequent paste will contain the text of the file:

$ xclip < /etc/passwd

Download source from http://people.debian.org/~kims/xclip/.