One thing to be very careful with wideners, is they they can badly mess up mono compatibility.

Especially some haas effect plugins, when in mono this can lead to horrible delay and phase effects.

I have found by far the most reliable way is to layers sound with wider material in the higher frequencies, fooling the brain into thinking the whole sound is wide.

There are reliable effects I have found, but again test in mono as some do not translate well.

The BX digital plugins work superbly, you can mono below a certain frequency and increase the side signal. In mono they have no negative effects either.

One of the best VSTs I have ever used is
http://www.brainworx-music.de/en/plugins/bx_digital_v2

None of this is a replacement for good layering and stereo placement imho.

There are myriad reasons why mono compatable sound is still very relevant in 2014, even on stereo club systems.