Visio is good, but not free. I believe it's only bundled with MS Office Professional (or at least it was way back when I first bought it).
Depending on what exactly you are trying to map out, you might find Bizagi a useful tool. Its focus is on Business Process Mapping, and so you won't find the same vast array of templates that you would see in Visio. However, you can download the Bizagi modeller software for free.
Many years ago (~15) , I also used a product called ABC Flowcharter, but I've no idea if it's still around. It wasn't quite as robust as Visio, but wasn't as narrowly focused as Bizagi.
Hope this helps.
SH813