Atmel (www.atmel.com)
I came across atmel because they made an inexpensive flash based 8051 product which could be programmed in circuit. It had been about 10 years since I had done anything with microcontrollers but I knew the intel 48/51 family pretty well. I had built a bunch of drawing robots around the chips when I started having a fight with the SDCC compilers optimizer (evelyn). I Decided at that point I wasnt going to use anything that wasnt supported by the gcc chain. A few months later I was using the AVRs. The chips are easy to use full of features and fairly inexpensive.
Avr Studio.
Like most manufacturers of microntrollers Atmel provides a relatively comprehensive set of tools on for free. Also like most manufactures these are only avalible on Windows. The development community has responded by creating several tools in the public domain. Examples of these include the gnu compiler chain (avr-binutils, avr-gcc, and avr-libc), several programming systems (avrdude, and uisp), and two compatible assemblers (avra and tavrasm).
There are a few pieces that are however trapped within the Avr Studio which would make life possible without windows.