I'm not using virtualization that much (every once in a while to test some software on different platforms), but I was a big fan of VMWare. Although some of it's components are a PITA to set up on Linux, it still works much better than QEMU (yes, with all the kernel drivers and stuff) and it doesn't require me to recompile my kernel like Xen.
However, there's a new kid on the block: VirtualBox. At first it didn't promise much, but that was only until I installed it...
Now, this is what I call USER FRIENDLY application. While I'm sure VMWare is great on Windows, on Linux its setup simply sux (in case you wish to only use freeware stuff). To get more to the point, VMPlayer and server tools are free. So, in order to use it like that, you can for example use QEMU's tool to create a blank .vmdk file and then run VMPlayer to install OS in it. Also, you need to answer a lot of useless questions during installation, and you need to extract host-tools (or whatever is the exact name, I forgot) from the server .iso file. But, that's not the main problem. First main problem is creating and editing .vmx file (by reading the instructions from the Internet?). Second main problem is creating the shared folders (i.e. a directory shared between guest and host OS). And the third main problem is switching between real CD/DVD ROM device and some .iso file (by editing .vmx file? Come on).
VirtualBox solves all these problems perfectly. Install is dead-simple. Creating a new image is only a few clicks in GUI (yes, even I'm a 'console' guy, I like slick GUI that makes stupid things - stupidly easy to do). Adding host extensions is a single click in the menu! Adding CD/DVD device or an ISO image is also dead-simple. And this brings me to shared folders. It is not hard to add one, but letting the guest OS know about it is not that easy. I had to look into manual for that one, and guess what: VirtualBox has one of the best application manuals I ever read. It's clear, concise and right to the point. It does not try to teach you how to turn on the computer like most of the 'generic' manuals out there, nor it tells you what's on the screen (we're not blind!). It gives the information a regular user would need, questions that might actually get asked.
Now, as I wrote, I'm not a hard-core VM user, so it might lack some advanced features I don't use. But, for a simple "test how my application works on this or that operating system" type of job, VirtualBox is perfect. IMHO, it's the best VM product for the Linux Desktop.
Thank you Sun, for making this gem open source.