Here is a list of different platforms for multicopters. I don’t have much personal experience from all these yet, but I will write more when I know more.
Kaptain Kuk – KKmulticopter
This is the one I began with to get started with my first multicopter project. It has the basic features and it doesn’t cost too much. You can buy it from those providers KKmultcopter.com, KKmulticopter.kr and RC-explorer.se.
Mikrokopter
This is a common platform with a huge community. I have the impression that it’s a solid and well tested multicopter platform. It uses it’s own motor controllers though, and not regular ESC’s. It’s a german site but a lot of the content is available in English too. Check out mikrokopter.de.
Arducopter
This is a Arduino based open source project with lot’s of possibilities and advanced features. Aeroquad among others is based on this platform. Check out the Arducopter site.
Hoverfly
I don’t know much about this platform from Hoverfly Tech. It has some good features and is used by e.g Brothers Olsen.
Commercial multicopter platforms
There are also a few commercial multicopter platforms that I want to mention.
Draganfly
Draganfly is a commercial prebuilt system for multicopters. Draganfly seems to be built of quality components and is designed for professional usage.
AR Drone
Parrots AR Drone is a consumer multicopter that you control with your iPhone. You also have a FPV video downlink to your iPhone. It has a protective hull for indoor use and seems to be a good choice for beginners.