A service that will print high-quality stickers, business cards, and what have you out of images that you send to them. Their prices are quite reasonable, also.
A database of what kind of adhesives should be used to glue something to something else.
A suite of open source electronic design tools. Uses a GUI for schematic capture. Also capable of analog and digital simulation, generation of parts lists and export of PCB designs for fabrication.
Fritzing is an open-source graphical application for converting a project prototyped with an Arduino into a PCB schematic for manufacture. Designed for people already familiar with Processing to do what they need to do easily. Cross platform: runs on Windows, OSX, and Linux.
Make your own hackerspace passports!
Wikihouse is an archive of open source building designs contributed by engineers and architects around the world. Each design is peer-reviewed and analyzed by others to ensure that the designs are sound. Some of them have already been built and the results posted for further review. The design principles include being as easy to build by people with minimal formal training, as energy efficient as possible, and easy to modify to fit a particular purpose. All designs are made in Google Sketchup and can be cut with an automill if available.
A pair of books licensed in the Creative Commons (BY) which talk about the architecture of open source software. The core developers of four dozen projects talk about how their software is structured, and most importantly why. They were written so that F/OSS developers wouldn't have to learn by reinventing the wheel, instead there would be a reference.
There are two books, which you can read online and download for free. Or you can buy them as Kindle editions (the proceeds go to Amnesty International).
MPC is a company that prints custom decks of cards (54-234 cards per pack) in multiple sizes. You'll probably want to buy in bulk to get a decent price. People tend to use this service for their aftermarket Foo Against Humanity decks.
A website that lets you upload SVG files and it'll convert them into Bootstrap compatible glyphicons for use in your website. Also has a huge archive of icons that you can download and use.
A plugin for Bootstrap 3 that automatically implements accessibility best practices (such as being friendly to screen readers and color schemes).
This site collects CNC and laser cutter meshes that can be used to manufacture open source office and home furniture.
Google has open sourced their Material Design Icons Library. It includes icons in PNG, SVG, and CSS glyph formats for many devices, functions, and features. Spritesheets are included. The icons have a Creative Commons By Attribution-Share Alike v4.0 license, so feel free to download and use them.
The indieweb is a movement in which people own their data, and run their own applications rather than other people's walled gardens. content is there and isn't censored arbitrarily. federation with some or all services is done with interoperable-by-design protocols and data formats. User interaction (replies, likes, stuff like that) are also federated back to your site(s).
RepRap is an open-source (hardware, software, and documentation) desktop fabrication system which lets you construct arbitrary objects (or parts for assembling more complex systems). It's unique in that you can construct one yourself for about $400us, and it is fully capable of making duplicates of itself.
Just what the title says - merit badges for nerds!
A blog dedicated to C'thul'hu mythos-related props and other crafts.
An open source object scanner that exports the data such that it can be replicated on a 3d printer.
Send them a graphic of some kind and they'll do custom fabric of it for you. No minimum order, they even sell test swatches.
A website which lets you design circuitry online in your web browser (i.e., without having to (buy, download and install PCB engineering software). The online circuit designer contains a library of known, documented, and trusted subassemblies of circuits (like power supplies, amplifiers, and volume controls) that you can use as the framework of your circuit, but you have to add the rest. They plan on making it possible to automatically generate and e-mail all of the files a third party small-run manufacturer would need to etch, drill, and mail your circuit boards to you.
3777 links, including 206 private