These are my go-to links when playing on the server. So you understand that the name "Ultimate" isn't an exaggeration this doesn't even cover all the mods that are running on the Ultimate server, lol.
Minecraft - Wiki - vanilla wiki
Railcraft - Wiki - advanced railroading stuff
BuildCraft - Wiki - quarries, tubes, engines, etc
Industrial-Craft - Wiki - machines, tools, armor, power, etc
RedPower 2 - Recipes - machines, pneumatics & stuff
Mystcraft - Wiki - new realms
Forestry - Wiki - bees, trees and new machines
Thermal Expansion - Wiki - very cool machines and stuff
Advanced Solar panels - Thread - better solar power
Factorization - Thread - barrels, etc
ComputerCraft - Wiki- turtles!
Gravitation Suite - Thread - advanced tools/armor
Applied Energistics - Site - advanced storage
Steve's Carts 2 - Wiki - advanced mine carts (goes very well with Railcraft stuff)
XyCraft - Wiki - was annoying useless blocks, now fun blocks with a purpose! (big liquid tanks, light blocks, etc)
If you are just getting into this stuff or want something to give you some ideas then YouTube "Let's Play" videos are a good choice. Direwolf20 is one of the big ones for this and I am sure people will be adding links to some of his stuff in this thread too. Also, you can find specific targeted videos on how to build most stuff from the basic "Focus On" type ones that focus on one mod or even one block/machine to the advanced ones that show you how to build complex machines or use complicated processes.
I figure we were overdue for a thread to dump all of our combined knowledge in so started this one. Feel free to add to it.