Advanced Micro Devices; makers of the 32bit Athlon, Thunderbird, Duron etc product lines, and the 64bit Athlons and Opterons (see amd64). All these chips are x86 compatible.
Their CPUs are in general faster than the Intel equivalent at the same clock speed; they have taken to the horrible trend of labelling their CPUs with a 'performance rating'; ie an Athlon XP 1800+ runs at 1533Mhz.