The Package Installer is a hidden app present in everyones Vita with which you can install DRM free content like Demos and Apps. There are different known ways to open up this App but all of them make use of calling a support_uri containing the internal TitleID of the Package Installer "NPXS10031".
After Firmware 3.20 Sony added some security instances to URI calls and patched away the useability of the PKG Installer app on retail devices..
[Tutorial] How to create the PKG Installer Bubble (on all firmwares)
Older methods to call the PKG Installer via support_uri call
Tutorials & Guides Compilation
[Tutorial] Convert your Vita to an IDU Kisok Device (3.60 only)
[Tutorial] How to open the PKG installer on 3.18/3.20 (via hackinformer.com)
[Tutorial] How to use the PKG installer (via hackinformer.com)
[Video] How to use and install PKGs (via by HRG _)
[Guide] How to sniff .pkg links with your Vita (via hackinformer.com)
QCMA v0.3.6 by Codestation
[Archive] PKG Files (via hackinformer.com)
[Archive] PKG Files (via wololo.net)
Frequently Asked Questions
Why wont some pkgs install for me? ERROR: C0-14478-4
The .pkg you tried to install is either corrupted or not DRM free
How do I know which .pkg(s) are DRM-free and can be installed?
Everything that comes from "http://ares.dl.playstation.net/" should be good to go.
Can I backup my installed Demos?
Yes and they will work forever on your account. Even if you update!
Can we install custom PKGs?
No, only official ones.
Can we install PSP or PS1 content?