A bit of the standard:https://www.eznec.com/

The free version is used by many in the HAM community and also beyond. I tried it and found it usefull but for more complex antenna's with many elements you will run into some limitations and need to buy the professional version.


This one worked best for me: http://gal-ana.de/basicmm/en/

The free version has many capabilities and a bit less limitations than the fee version of EZNEC but I'm sure it will depend on your needs. To get Started you can have a look at this youtube video to see how you use the softeware to design a 40M antenna. 

There is also a professional version of the software called GAL-ANA but this seems overkill for amateur use. 


I recently found 4nec2 as an alternative: http://www.qsl.net/4nec2/

Have not tried it yet. please comment on it if you have used it.