EZNEC

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.

MMANA-GAL

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. 

4nec2

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.