VirtualBox

Pros: Free & Open Source.
Easy to setup & use.
Great for using different Operating Systems on a single hardware simultaneously.

Cons: For a free software there is no con - but Vmware probably has more ease of access.

Overall: VirtualBox is a great FOSS virtualization solution. We use it at work for testing a software compatibility to different environments.

Pros: VirtualBox has updates often, supports a multitude of platforms (Windows, mac, Linux, Solaris...), and has a great documentation.

Cons: The hiccup for me is sometimes the VirtualBox Guest Additions, which on some Linux distros, I have to manually download every time I want to use it.

Overall: Overall, VirtualBox is a free and excellent choice for virtualization software, that you should try.

Pros: VirtualBox is available on pretty much all the majors platforms including Windows, Mac, and Linux. I'm using the later and you can run pretty much all type of systems on it (DOS, Windows, Mac, Solaris, and Linux).
It's super easy to configure and use.
I haven't experienced any major issue so far in the years I'm been using it.
The bi-directional handling of the files and shared folders between host and guest machines is very easy and smooth.
It is also fairly easy to expand the disk capacity of a virtual system. I had done that a few time as Windows keep swelling on my guest machine.

Cons: One of the majors cons I have with VirtualBox (on Linux) is the handling of the VM VirtualBox Extension Pack. Everytime I have an update of VirtualBox (which I don't usually notice at first), it will require you to go on the VM website, download the new Extension Pack and install it before you can actually start you VirtualBox session. It doesn't take long but it's annoying and I don't understand why the Extension Pack is not included in the mainstream update anyway.

Overall: Overall, VirtualBox is a great tool for virtualization. It's free and works very smoothly. It's available on all major platform and support all sorts of OS. It's easy to configure and easy to use. I had used VMware in the past (which is a paid software) and VirtualBox is, to me superior on any aspect and it's free.

Pros: Open Source & Free to use.
Easy to install & configure.
Lots of customization options.
Good documentation.
Great community & regular updates.

Cons: Interface has become outdated.
Less beginner friendly than competitors.

Overall: VirtualBox is a great virtualization tool which helps us launch any number of virtual machines over our Operating system. Although it may be daunting for beginners to grasp its multiple features, it is the best open source & free Virtualization tool with a great community support.

Pros: It can virtualize a lot of operation systems including Windows, Mac, Linux and even Android.
There are many configurable options available including amount of memory, video memory, storage and more.
The virtualization works extremely well with no issues.

Cons: It's difficult to set the resolution to HD widescreen.
GPU passthrough is not supported, so it's not good for virtualization of tasks that need a GPU.

Overall: VirtualBox is the perfect solution to virtualizing old and new operating systems. Definitely recommend this to people who want to use an operating system without going through the hassle of installing it on actual hardware.