Adobe has posted a free public beta of Adobe Photoshop CS6! You can download it from Adobe. [Update: Because the final CS6 software shipped in May 2012, the beta software link now redirects to the Photoshop product page where you can download the free trial version of Photoshop CS6, or buy it.] That page contains the download link, important information, and links to resources for learning Photoshop CS6.
Lightroom users: Photoshop CS6 contains Camera Raw 7, which includes full compatibility with the new Develop module controls in Lightroom 4.
Remember, this is a prerelease version. Read the public beta page carefully, and back up any irreplaceable images before editing them with pre-release software. Have fun!