I get several user requests per day (I'm administrator & head of support in wordpress theme & plugin support forum) about plugin suggestions so I decided to create a tutorial post about this topic. Please be aware that it's my personal opinion. I know there're hundreds of plugins out there and every plugin has advantages/disadvantages.
I tested all plugins I mention below with free and premium themes – however I can't promise that they work with your theme(s). I'll maintain this list and I'll add new plugins I think you might be interested in. You can suggest plugins by posting a comment :)