List of Quad Core Single Board Computers

ARMv7 Quad Core

Raspberry Pi 2

• Broadcom BCM2836
• 4x A7 900MHz
• 1 GB RAM
• 0GB NAND Flash( MicroSD storage slot)
• 35$

Raspberry Pi 2 was released in February 2015. Best software support in this class.

Odroid C1+

• Amlogic S805
• 4x A5 1.5GHz
• 1GB RAM
• 0GB NAND Flash( eMMC4.5 flash and MicroSD storage slot)
• 37$

Odroid C1 was introduced to market in December 2014 for 35$.
About 40% faster than Raspberry Pi 2.
Since August 2015 there is a successor board Odroid C1+ with small improvements and large heatsink for 37$.

Radxa Rock Lite

• Rockchip RK3188
• 4x A9 1.6Ghz
• 1GB RAM
• 0GB NAND Flash
• 39$ (since March 2015)

After 20$ price drop also an interesting board. Should be even faster than Odroid C1.

Radxa Rock Pro

• Rockchip RK3188
• 4x A9 1.6Ghz
• 2GB RAM
• 8GB NAND Flash
• 79$ (since March 2015)

For extra $$, you get 2GB RAM and 8GB included flash memory which is faster then SD card.

H4 Merrii Hummingbird

• Allwinner A31
• 4x A7 1GHz
• 1GB RAM
• 70$

Merri makes the official dev boards for Allwinner. Because of little public support and the current price tag this is not very interesting for non professional developers.

ARMv7 Octo Core and Fast (A12,A15,A17) Quadcore

These are currently the high end ARM development boards.
At least until 64bit ARM8 boards enter the market.

Optimus Board

• Allwinner A80
• 4x A15 4x A7
• 4GB RAM
• 169$

The official board from Allwinner.

Cubieboard 4

• Allwinner A80
• 4x A15 4x A7
• 2GB RAM
• 125$

Released now in March 2015.

Radxa Rock 2

• Rockchip RK3288
• 4x A17
• 2GB RAM (4GB optional)
• 129$

 The Rock 2 is designed as SOM board and base board combination.

Firefly

• Rockchip RK3288
• 4x A17
• 2GB RAM
• 129$ on aliexpress, 140$ at geekbuying.

"official sale" since 10/31/14 on aliexpress. There is now also an English webpage with forum.

Hardkernel XU3

• Samsung Exynos 5422 2.0GHz
• 4x A15 4x A7
• 2GB RAM
• 179$

Interesting board with built-in energy monitor. Now discontinued and superseded by XU4 board.

Hardkernel XU3 Lite

• Samsung Exynos 5422 1.8GHz
• 4x A15 4x A7
• 2GB RAM
• 99$

This is "low-cost" version of XU3 with slightly slower clock speed, removed display port and removed energy monitor at an interesting price.
Interesting board with built-in energy monitor. Now discontinued and superseded by XU4 board. 

Hardkernel XU4

• Samsung Exynos 5422 2.0GHz
• 4x A15 4x A7
• 2GB RAM
• 0GB NAND Flash( eMMC5.0 and MicroSD storage slot) 
• 2xUSB 3.0 1xUSB 2.0
• 1xGigabit Ethernet
• 74$ (including 5V5A PSU)
• Cooling with fan

New board, announced in July 2015.
Seems to be to currently the best choice in this group of boards, because of best price/performance ratio.

NVIDIA Jetson TK1

• Tegra K1 2.3GHz
• 4x A15 1x A7
• 2GB RAM
• 16 GB eMMC
• 1x USB 3.0
• 1x RS232 serial port
• 1x Gigabit Ethernet 
• 192$

Interesting board because of "Kepler" GPU.