General
ITP Physical Computing tutorials
Several tutorials around Physical Computing
MAKE: How to make PCBs
Collection of resources and guides
Sparkfun Tutorials
Tutorials on electronics design including EAGLE and SMD soldering
OpenCircuits
Wiki for sharing open source electronics knowledge, schematics, board layouts, ports and parts libraries
MassMind
Wiki tech reference around learning to design and produce PCBs
Complete Beginners Guide to the Arduino
A lot of Arduino example projects explained in every detail
Physical Computing (Book)
An introduction to Physical Computing by Dan O'Sullivan, Tom Igoe
Making Things Talk (Book)
Practical Methods for Connecting Physical Objects By Tom Igoe
Getting Started with Arduino (Book)
An introduction to the Arduino open-source electronics prototyping platform by Massimo Banzi
Fashioning Technology (Book)
A DIY Intro to Smart Crafting By Syuzi Pakhchyan
Hands-On Introduction to Electrical Engineering Lab Skills
More in-depth electronics from MIT Open Course Ware
PCB Design
PCB Design Tutorial
A popular quick guide on learning how to design PCBs, by David L. Jones
Build Your Own Printed Circuit Board
Effective book that covers the full process from design to production, based on EAGLE, by Al Williams
PCB Manufacturing
Makezine: "PCB"
Lots of how-tos around manufacturing PCBs on your own
Instructables: "PCB"
Lots of how-tos around manufacturing PCBs on your own
Lady Ada: Etching In-House
Semi-professional etching tutorial
Wikibooks
German How-To "Platinen selber herstellen"
Glossaries
EDA terms
Terms used in Electronics Design Automation (which is essentially what Fritzing does) are explained at EDA Consortium, EETimes
PCB terms
Glossaries of terms used in designing and manufacturing Printed Circuit Boards can be found at Golden Gate Graphics, PCBpro, SunMan
Electronic symbols
Overviews of symbols for electronical parts used in schematic diagrams can be found at Electronics Club, Wikipedia (German)
Assembling & Soldering
Surface Mount Soldering Tutorial
A detailed how-to by Sparkfun
Soldering Basics
Short and with pictures
SMT Soldering
Another one, with good pointers to videos