There's a massive number of Raspberry Pi OSes available. Most of them are some kind of Linux flavor and general-purpose and some other OSes are available for specific purposes like media streaming or gaming and emulation.

It is now also available for free ( USB booted) on PC or Mac based on i386 processors. Works well and boots fast.
The minimum USB key size needed is 2 GB. Pi promotors are working on a HDD bootable version too.

The new free Pixel version of Raspbian (a variety of Debian Jessie) has a good Chromium web browser with Flash and Pdf handling included. Can live with little RAM: 1 GB is ok, more is super.

Raspberry Pi also has an available "Pi Store" which houses hundreds and hundreds of apps ready to download and use on a Pi.

Many aptitude packages are available for this device. Helping installs go quickly.

Has it's own magazine, also available in print with very helpful articles and project ideas.

Runs off a 5V smartphone charger and can also be powered using a battery pack.

Cases, cameras, etc.

It's super-convenient to have multiple MicroSD cards with different images and switch them on demand. Much better than re-installing and re-configuring an OS every time.

Good for lots of connectivity in projects. connects to keyboards and mice.

This SBC's wired connectivity is pretty complete. Users will get four USB 2.0 ports, HDMI, Ethernet, a 3.5mm audio jack, CSI (camera interface), and DSI (display interface).

Raspberry Pi 3 Model B can be also be used as a console emulator using an OS image called RetroPie. Setting up RetroPie the first time will only take about 30 minutes on average for someone who has never done it before, and even less for experienced users. You can emulate various platforms ranging from early ‘80s legacy consoles like the NES, to more recent ones like the Wii and PlayStation Portable.

Raspbian's main goal is to be used as a teaching tool for schools in CS classes. As such, it has multiple development tools pre-installed. It comes with a Python installation, Wolfram Mathematica and Java IDEs.

The Raspberry Pi is very affordable.

Raspberry Pi has the largest following of any single-board computer. The amount of guides, tutorials and software available for the Raspberry Pi is unmatched by any other competitor. A regular user has close to no chance to run into a problem that hasn’t been covered already. If a web search doesn’t yield any results, the users on the official forums are very responsive and will usually reply within a day.

You can easily turn any Raspberry Pi into a fully functional home media streaming station with its ability to stream 60Hz Full HD videos. You just have to hook the Model B up to a Full HD monitor (through an HDMI cable), decent speakers (the Bluetooth 4.1 connectivity will allow the use of the wireless ones). Then download applications (Netflix, YouTube, Spotify, and Kodi for example are all supported out of the box) and your home multimedia station is ready. Once built, you can even control the media station using an Android or iOS app.

Setting up a Raspberry Pi is easy enough even for people who have not installed an OS before. The official documentation is very detailed in explaining how to install the official Raspberry Pi OS, called Raspbian and even a beginner can follow it without a problem.

The Raspberry Pi model 3 has an onboard 2.4GHz 802.11n Wi-Fi which works out of the box as long as the latest version of the default operating system is installed.

The model 3 B has built-in Bluetooth support. This is very helpful when connecting multiple peripherals and when the number of USB ports on the Pi is not enough.

The Raspberry Pi 3 model B offers 40 GPIO pins to be used within their projects. This is a very reasonable number of pins for most projects that will need them. The special communication protocols officially supported are the following: IIC (Inter-Integrated Circuit), SPI (Serial Peripheral Interface) and UART (Universal Asynchronous Receiver-Transmitter). The GPIO functionality explained above allows the Raspberry Pi 3 Model B to be connected with various peripheral devices and, as a result expands the functionality of the board. For example, IIC and SPI buses can be used for attaching multiple analog to digital converters which can be used to “read” analog channels like thermal sensors, humidity sensors, CO2 sensors, etc. Meanwhile UART can be used for communication between multiple Raspberry Pi’s.

Uses a separate micro SD card, so the storage can be upgraded or even taken out to read from / install a new OS.

The Pi 3 can play 1080p videos at 60fps, which is a significant upgrade from the previous Pi 2 which ran at 30 fps. However, the built-in browser has problem with running YouTube videos at 60fps but by using another browser such as Firefox which is installed separately there should not be any problems with running videos at 60 fps.

Extensive documentation available for beginners and experts. All sorts of projects covered.

While older versions of the Pi have had some annoying performance issues, the latest version has had a 50%-60% performance increase from the Raspberry Pi 2. This makes it a pretty decent choice for an everyday PC and it can perform basic tasks like opening and editing documents or browsing the web without a problem.