After VirtualBox turned bad (everyone says it's much slower, and it can't even be installed on lots of OSs anymore), people are turning to Qemu, which is better anyway because it's FOSS.

It uses qcow2 files (currently), not .vdi files (which VirtualBox uses). You can 'migrate' .vdi files to become qcow files (use your old VirtualBox VMs in Qemu) but I haven't done that yet.

Install was not simple, but did work.

To make shared folders in the current Qemu, you have to go to Memory > checkmark on 'Enable shared memory', then click the button at the bottom of the list 'Add hardware', then go to 'File System' and here you keep driver as 'virtiofs' (this is just for LinuxHost-LinuxGuest, I've read, not for Windows). Set your source path (yes the actual path) to where you want your sharedFolder to be on your Host (create this folder also). For Target path, just put the NEWdIRECTORYnAME of the folder on your Guest (don't put the actual path to it, just the name). ... Now inside the GuestOS, do sudo mkdir path/to/your/NEWdIRECTORYnAME ... then (still inside the Guest, do sudo mount -t virtiofs NEWdIRECTORYnAME /path/to/your/NEWdIRECTORYnAME (the first name has to be the same as you entered in your Qemu settings a minute ago). Now it should be working on both sides.