While qemu + kqemu was sufficient for most tasks on Solaris hosts, VirtualBox tends to be faster and simpler to use.
Sneak peak and what VirtualBox can do for you:

VBox in seamless mode.

You get mouse integration, seamless mode, snapshots, shared clipboard and folders, Alt-Tabbing to Windows apps... Virtualbox has it all.
Grab the VirtualBox-opensolaris-amd64-1.5.51-r28040-beta.gz package, gunzip it and install the package as root:
# pkgadd -d VirtualBox-opensolaris-amd64-1.5.51-r28040-beta1
Of course, you'll need to add some libraries to your LD_LIBRARY_PATH:
solaris% ldd /opt/VirtualBox/VirtualBox | grep not
VBoxKeyboard.so => (file not found)
libqt-mt.so.3 => (file not found)
VBoxRT.so => (file not found)
VBoxREM.so => (file not found)
VBoxVMM.so => (file not found)
libX11.so.4 (SUNW_1.1) => (version not found)
VBoxXPCOM.so => (file not found)
Just grep the installed package contents database for the proper path, and export LD_LIBRARY_PATH
solaris% grep VBoxKeyboard.so /var/sadm/install/contents
/opt/VirtualBox/VBoxKeyboard.so f none 0755 root bin 57552 64233 1202698345 INNOvbox
solaris% ldd * | grep "not found"
[...]
solaris% LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/VirtualBox:/opt/VirtualBox/qtgcc/lib:/lib:/opt/sfw/lib
solaris% export LD_LIBRARY_PATH

Problem 1: the VBox.sh script relies on /bin/bash, change to /usr/bin/bash or /usr/bin/ksh.
Problem 2: VBoxDD.so needs a 64 bit libdlpi.so, but that's only been updated in OpenSolaris:
On Solaris 10 u4:
solaris% file /lib/libdlpi.so.1
/lib/libdlpi.so.1: ELF 32-bit LSB dynamic lib 80386 Version 1, dynamically linked, not stripped, no debugging information available
solaris% grep libdlpi.so.1 /var/sadm/install/contents
/lib/libdlpi.so.1 f none 0755 root bin 49784 28431 1168280598 SUNWcslr
solaris% pkginfo | grep SUNWcslr
system SUNWcslr Core Solaris Libraries (Root)
Easy fix :-). We download it from Nexenta!
http://archive.nexenta.org/elatte-unstable/base/sunwcslrsolaris% /usr/xpg4/bin/ar x sunwcslr_5.11.82-1_solaris-i386.deb
So we file the libs in data.tar - 64 bit:
solaris% tar -tf data.tar | grep libdlpi.so
./lib/amd64/libdlpi.so.1
./lib/libdlpi.so.1
./lib/amd64/libdlpi.so symbolic link to libdlpi.so.1
./lib/libdlpi.so symbolic link to libdlpi.so.1
solaris% tar xvf data.tar
solaris% file ./lib/amd64/libdlpi.so.1
libdlpi.so.1: ELF 64-bit LSB dynamic lib AMD64 Version 1 [CMOV], dynamically linked, not stripped, no debugging
Now we can start VirtualBox and run Vista on Solaris using virtualization :-).

If you want sound, install
OSS.


As you can see, apart from the poor graphical performance (as expected from Virtualization software), performance is very good. Especially disk I/O.

And for all the people who wish to convert their VMware and QEmu virtual machines to Solaris / VirtualBox,
read this.
Expect a FreeBSD version of VirtualBox soon.
Running Windows 2008 Enterprise and Vista Ultimate on VirtualBox:

If you want VirtualBox Guest Additions, just download a debian package or whatever, and use ar -x to unpack it. Grab the ISO and mount it in the virtual machine. You need them for AMD PCNet drivers (networking) on Vista and 2008 for example.
And, once VBoxGuest Additions are installed, you can work in seamless mode. It basically means the host and the guest Windows both appear on the same host desktop:


Shared folders:

Want to have some fun? Use VirtualBox virtual machines on ZFS with compression and snapshots :-). You'll get at least 1.5x compression with virtually no performance hit (less data to transfer == faster with a minor hit on CPU for compression / decompression).
Note: VirtualBox also released a 32 bit package for 32 bit x86 Solaris systems.