How can I add reminders via the command line? Once in a while it may be useful to add some reminders to Reminders.app from the command line — especially because they are synced to iCloud. Is there any way to do so?

A solution that does not rely on AppleScript is preferred: a) for performance reasons (probably silly) and b) AppleScript solutions often feel clumsy or overly verbose to me.
Here's another version that allows you to set the title, end date and time via command line arguments.

#!/usr/bin/env bash                                                                                                               
# Make a new reminder via terminal script                                                                                         
# args: remind &lt;title&gt; &lt;date&gt; &lt;time&gt;                                                                                                                                                                                 

osascript - "$1" "$2" "$3" &lt;&lt;END                                                                                                        
on run argv                                                                                                                       
    set stringedAll to date (item 2 of argv &amp; " " &amp; item 3 of argv)                                                               
    tell application "Reminders"                                                                                                  
        make new reminder with properties {name:item 1 of argv, due date:stringedAll}                                             
    end tell                                                                                                                      
end run                                                                                                                           
END    


So if you were to name this script "remind" and give it executing privileges (chmod 755 remind), you could do this:

$ ./remind "Go to grocery store" 12/15/2013 10:00:00PM