The Minecraft launcher is getting another overhaul, and you can test it before it goes fully live! As with previous test updates (game or launcher), it is encouraged to report any bugs you find, ensuring as smooth a launch as possible for everyone. In addition, Mojang's Dinnerbone has provided a helpful FAQ (seen below), to answer many questions you may have!
- Windows: https://launcher.mojang.com/mc-staging/download/Minecraft_staging.zip
- Extract it to somewhere before running. It will make a bunch of files in the same directory as the exe!
- OSX: https://launcher.mojang.com/mc-staging/download/Minecraft_staging.dmg
- Mount it, drag it to your application directory.
- Linux tarball: https://launcher.mojang.com/mc-staging/download/Minecraft_staging.tar.gz
- Debian/ubuntu package: https://launcher.mojang.com/mc-staging/download/Minecraft_staging.deb
- Arch Linux: https://aur.archlinux.org/packages/minecraft-launcher/
FAQ
"I get a problem moving Minecraft_staging.exe with error code 32, what do I do?"
Read the above please! Delete your current exe and redownload it from the above link.
"I found a bug. What do?"
Please leave a comment with the following:
- The launcher version number (The bottom left corner. See this screenshot for reference.)
- What you did, what you expected to happen, and what really happened. ("I clicked on play and thought it'd play. It blew up.")
- A gist of the following files, found in either your .minecraft directory or next to the executable:
launcher_log.txtnativelog.txtnativeUpdaterLog.txt(may not exist. It's ok if it doesn't!)
- Your operating system & version
"I didn't find a bug. What do?"
Tell us your feedback! Even knowing that it works at all is a very good thing. Like it or hate it, just let us know what you think 
"This is all too complicated and taking too long... when will it be released?!"
Sooooon!
If you find anything to report, be sure to drop a comment in the thread posted here!