Xilinx is one of the leading general-purpose FPGA manufacturers. It now produces high-density Virtex-5 and low-cost Spartan-3A FPGA families.
Altera is the other major player on the general-purpose FPGA market. Its newest devices include high-density Stratix-IV family and low-cost Cyclone-III devices.
Actel manufactures high reliability FPGAs for military and aerospace markets. They have less densities than their Xilinx and Altera counterparts.
Modern Actel devices include ProASIC 3 and Igloo Flash-based families, radiation-tolerant RTAX antifuse-based FPGAs and general-purpose Axcelerator antifuse FPGAs.
Lattice Semiconductor's main products are volatile LatticeECP2 FPGAs and LatticeXP2 FPGAs with built-in Flash modules.
Atmel mainly focuses on microcontrollers, not on the programmable logic. However, they have an FPSLIC family which combines AVR MCU core with some FPGA fabric.
The FPGA tutorial has been created by 1-CORE Technologies, an FPGA design services provider.