All mockups are licensed under the CreativeCommons Zero license (except for things that aren’t mine to license, like the fonts used, the Gnome portions, or the Android Paste icon). You’re welcome to experiment with them and use them for your own UI proposals. They’re Inkscape SVGs, so it’s best to use Inkscape to open them.
Named after a carcinogenic dye put on oranges to make them look nicer, this is a very basic mock-up of a dark-themed Impress.
A mockup for an Android 4 tablet (landscape-only). You’ll need the Roboto font.
A mockup for an Impress remote for Android phones.
An older mockup for Android smartphones (portrait-only). You’ll need the fonts Bitstream Vera and Roboto.
Mockup of the current LibreOffice on Windows 7. Uses the Vegur font.
I may be forgetting some fonts: if you find I’m forgetting something, add a comment below and I’ll fix it.
SVG mockup kits from other parts of the web.
This is a collection of mockups for Gnome. These contain various useful tidbits to add to your mockup (like Adwaita widgets, for example, which are found in “theming/widgets”).
A mockup kit for elementaryOS.
The official icon pack for Android 4.0 from the Android Design site.