Commercial Antenna Modelling software
Mac
For the Mac there is only 1 native Antenna Modeler CocoaNec; This software is available with Source-Code, and as XCode is also free, you can rebuilt it to run on your new Mac (It is quite easy to do this). But I am not sure for home much longer this solution will work.
Windows
Here there are more choices, and even better all are now free !
4Nec2
This is my favorite antenna modeller. The reason ? The evolve functionality is un-surpassed in the other choices.
It is not as fast as MMANA but the control you have over modelling your antenna more than makes up for this.
MMANA
A very simple to use piece of software, which works very well indeed. It is extremely fast when it comes to calculating results.
EZNec Pro
With the retirement of W7EL, he graciously donated this software to the community.
Should you want the evolve functionality that 4Nec2 has, then you need to purchase a copy of AutoEZ also.
When developing a model
Should you want to try a 40m Delta loop and the model in program X looks great - please please, double check in program Y. There will be a slight difference in the length of a wire 10.2m instead of 10.18561m. But check the SWR and the gain, as there can be some surprises depending on how the software treats radials, small wire, tight bends etc.
The best solution may be to get a copy of NEC5 ($110) for this is the most accurate modelling software currently available.