These are some links I've found useful. I had posted some of my favorites on another forum awhile back so I just did a quick cut and paste.
This is a good article on understanding loads and using span tables;
http://www.umass.edu/bmatwt/publications/articles/understanding_loads_using_span_tables.html
This one then goes on to understanding beams;
http://www.umass.edu/bmatwt/publications/articles/calculating_loads_on_headers_and_beams.html
The AWC joist and rafter spancalc;
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
The AWC joist and rafter calc is great, it comes up short when you need a beam, this is the same people's beam tables;
http://www.awc.org/Standards/wsdd.html
Also while on any of their pages click the "publications" tab on the left of the page, then click "free download library"
This is the connections calculator by the same folks who provide the joist and rafter calc. I imagine most of you know, connection failures are much more common than timber failures. This is how to check the strength of a nail or bolt;
http://www.awc.org/calculators/connections/ccstyle.asp
The American Plywood Association's website is here;
http://www.apawood.org/
Georgia Pacific's I joist and LVL span tables are here;
http://www.gp.com/build/documentviewer.aspx?repository=bp&elementid=4372
For Glulam beams the AITC has span tables;
http://www.aitc-glulam.org/capacity.asp
Click on their "publications" tab as well, lots of free technical info
Residential steel tables can be found here;
http://www.toolbase.org/Design-Construction-Guides/Exterior-Walls/steel-beam-column-load
For flitch plates this is a good set of tables;
http://www.toolbase.org/PDF/DesignGuides/flitchplate.pdf
When all that runs out I back up and punt
The AWC also publishes the "Design Values for Wood Construction", the allowable strength values for wood by species and grade. I have a little sawmill and use timbers that you don't typically find at Home Despot. I'm building a horse barn soon out of tulip poplar, I can find the typical strength value of that here;
http://www.awc.org/pdf/2005-NDS-Supplement.pdf
Kinda funny how our grandads used to use the trees in the backyard and now I'm supposed to use stuff from Estonia and Lithuania
I've taken some basic engineering formulas and plugged them into simple javascripts to crunch the numbers. These are some calcs I've written, click the beam and column tab and it'll open to that group, you should of course have your engineer check the results, but they work for initial design;
http://www.windyhilllogworks.com/CalculatorIndex.htm