Below are several extremely useful tutorials that have greatly helped me advance my knitting skills.

Judy's Magic Cast-On - The cast-on I use exclusively whenever my project needs a provisional cast-on.
Cable Cast-On - Great cast-on for beanies, or other projects where you want a simple, clean edge.
Long Tail Cast-On - Probably the most popular cast-on, and usually the one taught to beginners. Very stretchy. Good for projects with ribbing.
Tubular Cast-On -Very clean and professional looking. Starts project with 1x1 ribbing, Great for sweater bands and sleeves, but be careful not to cast-on too tightly.

K2tog - Knit two together. Slants right.
SSK - slip, slip, knit. Slants left, in the opposite direction of k2tog.
Double Decreases - there are several shown in this tutorial, but I'm partial to the centered double decrease.

Knitting in the Round:
On double pointed needles.
On circular needles.
On two circular needles.
Magic loop.

Finishing Techniques:
Mattress Stitch
Duplicate Stitch
Fancy Alphabet Charts for Knitting

Disclaimer: I did not create these tutorials or techniques. They have been made available for free on the internet.