Useful sites as a Hardware Engineer

While at work, I'd use a number of different websites to help me find out what I need or how I can find it out. Enough can't be said for just googling what you're looking for. Wikipedia is full of useful information as well, for example RS232 pinouts, acronym descriptions and details.
If you're having specific trouble with finding information on a component, check the manufacturer's website, more often than not, they have all the information you need.

I've listed below a few simple enough websites which I use on a day to day basis and they're very useful and easy to use.

Mega Converter
Handy tool for converting measurements, I usually use it for naming footprints for IC's.
http://www.megaconverter.com/mega2/

Standard Resistor Value Calculator: 1%, 5%, 10% - EIA: E12, E24, E96
This is a handy tool to look up what resistance values are possible with regards tolerances.

For parts ordering, I'd recommend the following
Radionics, based in Rialto, ship same day, cheap enough, however they don't have the best range of components. Very handy if you need something quick and they have it. Website is easy to use.

Farnell Electronics Ireland, these guys have a great range of components and tools. They're fast and efficient, but slightly more expensive. They lack access to datasheets or specific details on the components but trying to find what you need is easy enough.

DigiKey Corp., based in the US, these guys have everything, even the most obscure of stuff. They are fast and efficient. Shipping usually costs a bit more because they're based in the US and setting up an account makes it a little more difficult if you're just buying for personal use. Other than that, Digikey make life so much easier. For every component, they provide easily accessible datasheets and nearly all the information you need.

Mouser Electronics, UK based company, I've not used these guys all that much, but they do have a great selection of components and would be on my list of vendors for purchasing components.

For Learning, I'd recommend the following
cprogramming.com A website designed to help learning C or C++. Understandable C and C++ programming tutorials, compiler reviews, source code, tips and tricks.

Circuit Design Lab, a handy website to help you brush up on basic circuit design with java program to help demonstrate change in circuits.

Prentice Hall Interactive Online Resources, includes complete training courses and CD's for purchase.

I can't think of anything else off hand now, but this looks good for the moment. I think I'll post up a useful programs post next.

List of commonly used manufacturer sites include:
Advanced Micro Devices
Analog Devices
Atmel Corporation
Foxconn
Intel
Integrated Device Technology
Linear Technology
Maxim IC
Micron
Numonyx
Samsung
Silicon Storage Technology
ST MicroElectronics
Texas Instruments