CTRL-U - kill from the cursor to the start of the line
CTRL-K - kill from the cursor to the end of the line
CTRL-W - kill a "word" or "path component" behind the cursor
ALT-D - kill a whole word after the cursor
See also: help editor
Killing means deleting text and moving it to the kill ring. This is a lot like the "cut" operation in graphical applications. The kill ring is thus a bit like a clipboard, but with a twist: a log is kept of previous kills and you can "rotate" the kill ring to paste older entries.
CTRL-Y - paste the latest entry in the kill ring
ALT-Y - rotate the kill ring and change the latest paste
See also: help killring
Do you sometimes type a command, only to realize you need to run some other command first? Perhaps you use CTRL-C to abort the current command line and type the new one? Now you need to type the previous one all over again, as it's not in the history! Perhaps you've resorted to CTRL-A # ENTER to comment out the commandline and put it in the history? There's a better way! Just hit CTRL-U, run the other command, then CTRL-Y to get back the previous commandline. If you cluttered the kill ring while entering the other command, just rotate with ALT-Y until you get to the right one.
Bonus feature! If you're running an X server, fish integrates the kill ring with the clipboard. This means you can easily cut-and-paste from fish to graphical applications (for example CTRL-U in fish and then CTRL-V in the other application), and you can copy-and-paste from graphical applications to fish (CTRL-C in the other application, CTRL-Y in fish).
In fact, this isn't just convenient - it's more secure! The following demo works with fish if you paste with the mouse or the terminal (for example CTRL-SHIFT-V in gnome-terminal), but if you paste with fish, that is with CTRL-Y, the evil command will be inserted into the commandline but not executed until you press ENTER:
If you liked this post and would like me to write about more tips and tricks, let me know and get fish users you know to join this community. It's something of an experiment to see if there's enough Google+ users interested in fish to invest any time in making this an active community rather than just a badge.