I Love PDF

I Love PDF - converter (in browser)

Merge, split, compress PDF
Convert PDF to/from Word, PowerPoint, Excel, and JPG
Watermark, rotate, and unlock PDFs

Click here to go directly to the features list.

Blink

Blink

Rendering engine for Chromium (Google Chrome browser) and Chromium OS (Google Chrome OS).
Open source community development.
Tons of resources available for interested browser and operating systems developers.

Sumatra PDF Reader for Windows

Sumatra PDF Reader for Windows

Reads these formats: PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader
Comes in 32-bit & 64-bit versions
Full install or portable
Works on these Windows Versions: XP, Vista, 7, 8, and 10

*I've had really good luck using Sumatra PDF Reader on PDFs that Adobe Reader cannot open.

pfsense Open Source Firewall

pfsense Open Source Firewall

This open source firewall project is hosted by Netgate.
Works with varied hardware and in the cloud.
Offers filters, routes, firewalls, scrubing, and more.

Source code is on github.