Canonical has already announced the beginning of the development cycle of Ubuntu 13.04 codenamed Raring Ringtail. Since there won't be any alpha or beta releases (there will be just one beta release before final release), eager users can try it out by downloading the daily builds from the Ubuntu CD image site.
If you are planning to download daily builds of 13.04 don't expect many changes. What you will get is to try out new features as and when they are added to the OS - it will give you an idea of how 13.04 is shaping up and you can, as usual, help the project by filing bug reports. Excited users can test it by downloading the images from here and installing it on their testing machines. Just keep in mind that these are test builds and you may end up with a broken system, so don't try it on production machines. Since the builds are still in heavy testing and modification stage, one may face regressions and unstability including crashes and loss of data. Try it at your own risk.
Since Canonical has decided to drop Alpha releases and will release just one Beta before the final release these daily builds are the only way for users to test the upcoming releases.