Probably the most popular Arduino library for programming these LEDs is FastLED. It features a ton of useful functions and super optimized routines. And you cannot only use LEDs with the WS2811 controller, also SPI based controllers. However, I failed getting it to work with a 8 MHz AVR. It seemed like the timing was way off.
Edit: It is the ATmega 1284P that will cause problems with FastLED. Apparently the bit-banging-routines are only working on 328, 32u4 and 2560. This is due to the larger memory the 1284P can access, which needs 3 address bytes instead of 2 with the 328. Therefore, jump-instructions take one tick longer. At 8 MHz that is 0,125 µs which is 10% of the period of one bit at 800 kHz (the clock frequency of the WS2812). That's why the actual clock frequency of the AVR needs to be 10% higher than defined in the F_CPU macro. According to this, the developers of FastLED are aware of this issue, but are not going to fix it.
Over at the Arduino forums, someone recommended using a light weight WS2812 library instead. I tried this one and it worked like a charm. Unfortunately, it is so "light" that it doesn't even support a global brightness parameter. Also, the way the HSV to RGB conversion is a bit user-unfriendly by using an indirection with another RGB-struct.
So I changed the whole library by Tim aka cpldcpu, Matthias Riegler, Windell H. Oskay and Freezy a bit up. I added direct RGB and HSV access to each pixel, the optimized HSV to RGB function from FastLED and a global brightness method, which is defeatable by commenting out a single line (to make it lighter for ATtiny). It isn't beautiful, but it works and some of you might be interested in it, so I shared it.