Computers with Ubuntu preinstalled
You can find lots of hardware suppliers here who sell machines with Ubuntu preinstalled - https://help.ubuntu.com/community/UbuntuPre-installed
How can I find out if my hardware is supported?
First, try the live DVD
To find out if your hardware is supported there are numerous ways to do it. The first thing to do, if you haven't installed Ubuntu Studio yet, is simply download the Ubuntu Studio live image, boot it, and see how things go. Since most drivers are already included with the live image you will most likely not need to install anything to find out. Instructions for how to boot the live image can be found here.
Note that you will often need to properly configure your software to have your device running properly. For audio devices, have a look at Introduction to audio
Most common network devices work out of the box on any Ubuntu flavor. Below are link(s) to pages with more information about network devices.
WifiDocs - Community maintained Ubuntu wiki section about wifi
Generally, most pci cards are well supported. To see a list of supported devices see http://www.alsa-project.org/main/index.php/Matrix:Main
To see a list of pci-e cards supported, see http://www.alsa-project.org/main/index.php/Matrix:Main
Most stereo devices will work out of the box (unlike multichannel devices), but often not with settings above 48kHz/16bits. This is due to most devices not following the 2.0 usb class definition, while most devices still adhere to the usb 1.1 class definition. With usb1.1 the performance is much more limited, and therefore the number of channels, as well as the samplerate and bits range are limited. A few devices stand out, by either having device specific firmware available, or being usb 2.0 class compliant.
Note that the devices below do not make a complete list of fully supported usb devices by any way.
Presonus 1818VSL (supported with the newer versions of alsa) See http://lists.linuxaudio.org/pipermail/linux-audio-user/2012-May/084993.html
Focusrite Scarlett 2i2 - USB 2.0 Class complient and Fully supported out of the box with ALSA (snd-usb-audio module). See http://wiki.linuxaudio.org/hw/focusrite_scarlett and http://focusritedevelopmentteam.wordpress.com/2012/04/23/linux-and-focusrite-novation-products/. Article confirming that this device is fully operational: https://community.ardour.org/node/4709. Specs available at http://us.focusrite.com/usb-audio-interfaces/scarlett-2i2/specifications
Focusrite Scarlett 2i4 - USB 2.0 Class complient and Fully supported out of the box with ALSA (snd-usb-audio module). The 2i2 and 2i4 are both confirmed to work well with Linux, ALSA and JACK. Using the correct settings, you can achieve near 1ms latency with no xruns using 96Khz or 48Khz 24 bit audio with the Scarlett 2i4. See http://wiki.linuxaudio.org/hw/focusrite_scarlett Article confirming that this device is operational: http://www.linuxmusicians.com/viewtopic.php?f=6&t=10142.
M-Audio MTRACK - Class complient. This audio interface is based on the IO2 Express made by Alesis (they look identical in appearance and share identical specs). Specs available at http://m-audio.com/products/en_us/MTrack.html.
NI Komplete Audio 6 - Class complient. See http://wiki.linuxaudio.org/wiki/hardware_support.
Supported, but may not work well at lower latencies:
M-Audio Fast Track Ultra - see http://www.alsa-project.org/main/index.php/Matrix:Vendor-MAudio
M-Audio Fast Track Ultra 8R - see http://www.alsa-project.org/main/index.php/Matrix:Vendor-MAudio
Currently, a select number of devices are well supported for pro audio, using the firewire driver with jack. To see the list of supported devices, checkout http://ffado.org/?q=devicesupport/list.
Most graphic cards have very good support out of the box. Upon installing Ubuntu Studio, if the default driver is not good enough for your needs, you will have the option to install a non-free proprietary driver, if such are available.