What are some tools available for drawing good figures for computer science research papers and presentations
Personally, I am a fan of the TeX-Package TikZ. It is easy to draw simple diagrams, include small pictures or create structures by using loops and conditionals.
It also supports relative positioning, which makes everything a lot easier than manual placement.

Since all of the input is plain text, it works great in combination with source control (e.g., git) and prior versions can easily be reverted back to.

I have to mention it takes some time to get used to the syntax, but the folks over at the TeX Stackexchange are usually very helpful. They have a great showcase of scientifc pictures, too! https://tex.stackexchange.com/questions/158668/nice-scientific-pictures-show-off

When images from other documents are to be included, I usually crop them out as vector graphics from pdfs by using Libreoffice Draw. With both tools in combination, I often get satisfying results since everything remains vector graphics for the entire diagram if i wish to.