Today Eagle is the most popular electronic design automation(EDA) application for PCB design. Eagle is becoming a common language for PCB design among engineers, designers, hobbyists and students.
These are the reasons behind its popularity:
1. Libraries with thousands of components: You can find almost any kind of components in Eagle libraries collection. In case you don’t find one, you can create yourself. There are several tutorials online.
2. Loved by open source hardware community: It is widely used by most of the open source communities. So, Eagle is becoming a common language or software among open source hardware enthusiasts. 100% boards and modules released by Arduino, Adafruit and Sparkfun are designed using Eagle.
3. It is free: You can get a freeware version of Eagle. With a freeware version you can design almost any type of modules and boards for Arduino and Raspberry Pi. It includes 2 schematic sheets, 2 signal layers, and 80 cm2 board area. You can install it on Windows, Linux and Mac computers. Almost all the boards designed by Adafruit and Sparkfun can be designed by the freeware version of Eagle. Download Now.
4. Design files are available for free: It is used by companies like Adafruit, Sparkfun, Seeedstudio, etc. Several engineers and hobbyists have used the free Eagle files provided these big companies. You can literally use those eagle files to manufacture your own circuit boards or kits. You just need to find a PCB manufacturing company.
5. Accepted by PCB manufacturers: Most of the time, manufacturers look for PCB gerber files extracted out of a PCB files. If you are not able to create your own Gerber files, the manufacturers can extract the Gerber files from your Eagle *.sch and *.brd files, you just need to send them for manufacture. Most of the companies will accept Eagle files. There are so many other PCB design software which are not accepted by several PCB manufacturing companies.
Here is a list of popular Eagle libraries made by Sparkfun, Adafruit, Arduino, etc. You can simply use the libraries to use the components and draw you schematic.