The second alpha of Ubuntu 15.04 has been released. Here the announcement:

Pre-releases of the Vivid Vervet are *not* encouraged for anyone
needing a stable system or anyone who is not comfortable running into
occasional, even frequent breakage. They are, however, recommended for
Ubuntu flavor developers and those who want to help in testing,
reporting and fixing bugs as we work towards getting this release
ready.

Alpha 2 includes a number of software updates that are ready for wider
testing. This is quite an early set of images, so you should expect
some bugs.

While these Alpha 2 images have been tested and work, except as noted
in the release notes, Ubuntu developers are continuing to improve the
Vivid Vervet. In particular, once newer daily images are available,
system installation bugs identified in the Alpha 2 installer should be
verified against the current daily image before being reported in
Launchpad. Using an obsolete image to re-report bugs that have already
been fixed wastes your time and the time of developers who are busy
trying to make 15.04 the best Ubuntu release yet. Always ensure your
system is up to date before reporting bugs.

This alpha features images for Kubuntu, Lubuntu, Ubuntu GNOME,
UbuntuKylin and the Ubuntu Cloud images.

Kubuntu:

Kubuntu uses KDE software and now features the new Plasma 5 desktop.
The Alpha-2 images can be downloaded at:

http://cdimage.ubuntu.com/kubuntu/releases/vivid/alpha-2

More information on Kubuntu Alpha-2 can be found here:

https://wiki.ubuntu.com/VividVervet/Alpha2/Kubuntu

Lubuntu:

Lubuntu is a flavour of Ubuntu based on LXDE and focused on providing
a very lightweight distribution.
The Alpha-2 images can be downloaded at:

http://cdimage.ubuntu.com/lubuntu/releases/vivid/alpha-2/

More information on Lubuntu Alpha-2 can be found here:

https://wiki.ubuntu.com/VividVervet/Alpha2/Lubuntu

Ubuntu GNOME:

Ubuntu GNOME is an flavour of Ubuntu featuring the GNOME desktop environment.
The Alpha-2 images can be downloaded at:

http://cdimage.ubuntu.com/ubuntu-gnome/releases/vivid/alpha-2/

More information on Ubuntu GNOME Alpha-2 can be found here:

https://wiki.ubuntu.com/VividVervet/Alpha2/UbuntuGNOME

UbuntuKylin:

UbuntuKylin is a flavour of Ubuntu that is more suitable for Chinese users.

The Alpha-2 images can be downloaded at:

http://cdimage.ubuntu.com/ubuntukylin/releases/vivid/alpha-2/

More information on UbuntuKylin Alpha-2 can be found here:

https://wiki.ubuntu.com/VividVervet/Alpha2/UbuntuKylin

Ubuntu Cloud

Ubuntu Cloud images can be run on Amazon EC2, Openstack, SmartOS and
many other clouds.
The Alpha-2 images can be downloaded at:

http://cloud-images.ubuntu.com/releases/vivid/alpha-2/

Regular daily images for Ubuntu can be found at:

http://cdimage.ubuntu.com

If you're interested in following the changes as we further develop
Vivid, we suggest that you subscribe to the ubuntu-devel-announce
list. This is a low-traffic list (a few posts a week) carrying
announcements of approved specifications, policy changes, alpha
releases and other interesting events.

http://lists.ubuntu.com/mailman/listinfo/ubuntu-devel-announce

A big thank you to the developers and testers for their efforts to
pull together this Alpha release!

Walter Lapchynski, on behalf of the Ubuntu release team.

  Ubuntu 15.04 Alpha 2 Released