Once you start measuring the rms component of the sound it can be displayed on a meter of some sort. With many DAW's it will appear as some indicator but low down on the scale. It is harder to see down there and Sonar shows true rms which puts the indicator 3 dB lower again.  Things are more difficult to see clearly down at -14 dB (-17 Sonar) or -20 dB. (-23 Sonar)
 
The VU meter has a nice indicating system with a scale and the needle which shows a high FSD at 0dBu when the reference voltage is reached. There is 3 dB above the reference level on the meter which is adequate for showing signals that are starting to get a little high. It is very easy to see any indicator high up on a scale. And it moves with a ballistic movement which I firmly believe tells you a lot about whatever it is showing. A track, a buss or the stereo master.
 
Real meters are very nice and I have built a very nice set. But the VST's are getting great in my opinion. They use little or no CPU resources and can be placed anywhere in the signal chain either in mono or stereo. I prefer the VST to show the original VU style indicator. There are many other GUI's that are not based on the original VU meter.
 
http://www.bluecataudio.com/Products/Product_DPeakMeterPro/
 
While they are good they don't show as much as the ballistic stuff does. But I guess the ballistic behaviour of something the BlueCat meter can be learned. This display I must say is a class act and the movement of the meter can be observed for sure. I just feel the standard VU meter movement is very easy and nice to observe. The Klanghelm meters behave very similar to their hardware counterparts.
 
http://klanghelm.com/VUMT.html
 
They are cheap, install easily, can be calibrated to any reference level of course and offer many adjustments to the ballistic behaviour. There are alternate skins. The mono meter can sprout an extra needle and show two channels at once! Very cool. (You can see much more clearly the differences between left and right channels this way) The PSP meters look the part but I found the fall behaviour a little different to the real thing. Unless they have got it better. I maybe need to spend more time tweaking the ballistic behaviour a bit more.
 
http://www.pspaudioware.com/plugins/tools_and_meters/psp_2meters/
 
I have devised a test that shows the ballistic movement. Rise and fall response. My real VU's are in close proximity to the bottom of my computer monitor so I can see all of them at once. You can almost mix using a VU meter. (many highly respected engineers have done it) Bringing drum levels up to a certain mark and bass and so on. You learn to allow for many things coming in. Adding lots of things to buses or the stereo buss especially where the mix sounds great and the VU's are just hitting 0 dB nicely. And not bouncing over at all. Dancing the right way. When you observe beautifully crafted mixes and mastered mixes the VU's dance a certain way. When your mixes start to sound the same the VU's move the same. When the dynamics on a track are even a little out of control or the mix on a buss or the stereo master sucks, the VU's tend to swing wildly and they don't dance well.
 
When all your track, buss levels are just reaching 0 dB VU nicely everything is running at a perfect gain structure. No clipping anywhere, plenty of headroom above your rms component, least distortion anywhere because you are sitting well below any possible clipping points which could dirty your precious signal anywhere. It is the way it was all done before but for some silly reason modern DAW's have dropped it all together but it is easy to put back in. And we need our fast peak reading systems too to keep an eye on fast transients that are not going to make the VU move very much. (individual drum sounds that is but total drum mixes end up moving the VU very nicely)
 
Both things (peak and rms) together work perfectly with each other.