S3 holds the S3TC patents. You already boycott them just by not knowing about their products. Microsoft has the ability to sub-license S3TC with their DXTC technology; you cannot avoid this fee, as it is included in the cost of every video card you have ever purchased.
SGI holds floating-point texture and framebuffer patents. Like S3, you don't actually buy anything they make.
If you're feeling proactive, you could attempt to boycott Sorenson, holder of h.264, by refusing to buy anything with h.264 abilities, or Apple, with whom they are deeply linked. Again, this ability is in most discrete video cards, so a boycott will not work.
I senses a pattern! Let's just boycott every company beginning with the letter S. That'll teach them!