Before printed-circuit board (PCB) was invented, electronic assemblies used point-to-point connections, wires, and bus bars. Today practically every electronic product is constructed with PCBs, which hold the components and provide the interconnections between them. Unfortunately, good PCB design isn't something that's taught in school. They may teach how to use CAD and other software, but not how to do a design that takes into account criticality of components location, non-zero trace impedance, the coupling of signals from one circuit to another, radio transmission, temperature rise and other physical effects. You can make right connections between all the components, but the circuit may not work as required. This site will tell you the important things to know about designing of PCB and provide layout guidelines, online calculators and other info.
An introduction to printed circuit board design- basic guidelines and links to free software downloads, trace calculators and other online resources.

Online trace width calculator based on new IPC2152 with comparison to legacy IPC2221 and an equation that interpolates IPC curves.

Calculator of trace spacing for internal and external layers per IPC2221B and IPC9592B.

Basic layout guidelines for switching power supply boards.

PCB design jobs- current list with daily updates.

Electrical creepage calculator per UL60950-1 2nd Edition.