The ability to use PHP 4 style constructors is getting removed from PHP.Without an update, many plugins will eventually no longer work (this is PHP breaking this backwards compatibility, not Word Press) One of the more common uses of the PHP 4 style constructor (as opposed to PHP 5 style Note: Starting in Word Press 4.3, regardless of the PHP version in use on a server, Word Press will throw a deprecated notice when one of the PHP 4 style constructors is called specifically for widgets.
If you see a friend's plugin on this list, let them know as well.
Friends don't let friends use PHP 4 style constructors.
It's possible this list isn't complete, so please double check all your plugins that are making widgets to make sure they don't needed to be updated.
This list was compiled using the current stable version of all (at the time) 51,501 plugins on Word as of 5/25/2015 Update (7/1/2015): There's a new make.article on this topic.
Update (7/3/2015): Hattip to George for pointing out a way to find more plugins that need to be updated.
List is now updated and there's now 4,004 plugins on it.
I also added plugin titles, and author names and contributor username.
Instead of reading through all 4,004 plugins on this list, try control/command + f for your Word username, your first + last name, and any plugin titles you've written. Plugins who use a PHP 4 constructor that do not call either of those also need to be updated, but we don't have a list of those.
Check all your plugins for PHP 4 style constructors, especially your plugins that have widgets.