Inkscape is a very good program for making figures and illustrations, it's open source, and free.

Ya, I agree wit Samuel. Its fantastic drawing tool especially when combine with Chemdraw.

Have You thought about Blender 3D? It is also open source software, quite simple in use. You can obtain really interesting results.

look also here https://www.researchgate.net/post/Can_anyone_recommend_a_good_tool_program_for_drawing_biological_diagrams

I have always found a pencil in my own hand is best!

It is also the cheapest!

Adobe Illustrator is great. You may also find some images already available at a great resource: http://www.somersault1824.com/

I agree, Inkscape is great vector program ;)

If You think about vector software I can recommend Xara.

Did you try Adobe Ideas on Ipad?

Inkscape for 2D and Blender for 3D pictures.

Blender for animations

For instance: https://www.youtube.com/watch?v=RasemoYzsFA