These are the major sites I use for any reference material or parts ordering.

Of course, there is also Google...

Stamps

Parallax

I2C:

Philips

USB:

FTDI - makers of the FT232 USB to serial and FT245 USB to parallel chips

Futurlec - NZ/Australia suppliers of FTDI chips, and much much more.

Semiconductor Manufacterers:

Texas Instruments

National Semiconductor

Philips

Maxim IC

LCDs:

Hitachi - for information on how to connect to HD44780 based Parallel displays

Matrix Orbital - a large range of serial character and graphic LCD displays

Parts suppliers (in New Zealand):

Futurlec

RS Components

Dick Smith Electronics

Power Technology - Maxim IC and Dallas Semiconductor. NOTE it is worthwhile bookmarking this url! Maxim IC/Dallas Semiconductor provide FREE samples of their chips!

Jaycar Electronics - Huge range of Electronic components.

Just Junk - for those hard to get or bargin parts

Parts suppliers overseas & General Information:

Hobby Engineering - The WEB's newest source for robotics supplies and information

Wizard Hardware - The #1 information site for all you programming and hardware needs