Cullen Vance shared this great list of free open source software to do the thing you want! Here’s what he had to say about it:

“Share for an artist friend in your life! So, it wasn’t until recently, through a conversation with a good friend, that I realized I have the very strange hobby of collecting the best free/open source creative software on the web. So, here is my gift to all the artists out there. This list is the culmination of 6+ years of scouring and testing. I have spent hours in most of the programs, and many of them I use in every single project I work on. Many of them also have Mac versions! If this enables even just one person out there to start making what’s in their soul, then all the work has been worth it. Also! Bonus prize is the collection of wonderful websites that have saved my life a couple of times! **Oops! Should be Sculptris not Sulptris.”

Source: Cullen Vance