Update: The ISOs are officially available now and the links published below are the correct ones.
Ubuntu 15.04 Vivid Vervet has been already announced but the ISOs are still not available. It seems, however, that you can download the 15.04 ISOs following the naming pattern Canonical uses for hosting ISOs. The following links are live:
- Linux kernel 3.19 (customized)
- Systemd replaces Upstart (you can fallback to Upstart by installing the upstart-sysv package)
- Unity 7.3 and Compiz 0.9.12 combination
- Several minor but significant visual changes in the desktop (like Always Show Menu option)
- Firefox 36 & Chromium 41
- LibreOffice 4.4
- Ubuntu Make (aka Developer Tools Centre)
Note that we have not tested these ISOs yet and cannot certify that these are the right ones. The download availability announcement has not been done at the time of writing (but will happen anytime soon!). Download and test at your own risk.
To get extra mileage from Ubuntu, check out this article. Many of the tweaks still apply.