Microchip Info on pic microcontrollers and other Microchip products.

Analog Devices Take a look at the AD9xxx DDS.

National Semiconductor More pdf data sheets.

Philips Semiconductors Good selection of data sheets for discrete components and i.c.'s

Siliconix FET and MOSFET data.

Minicircuits Mixers, Amp's, Transformers, etc.

Amidon Ferrite and Iron powder cores.

Micro Metals More Ferrite and Iron powder cores.

TOKO Inductors.

Coilcraft Inductors, Magnetics, Filters.