Besides our own Effective Skiing website there are a few websites with advice, tips and drills. As usual online, use your own judgement...
ski smarts pretty cool drills, lessons and problem corrections.
There's a lot to be learned from interacting with others, instructors, coaches or just other interested skiers on forums like:
YouCanSki.com collection of interesting articles on advanced topics
www.dartfish.tv/ussa - the USSA Center of Excellence video channel - well done, USSA!
This is not strictly an online resource, but you can order a large selection of technical material from the USSA: alpine.usskiteam.com/alpine/education-shop.
Some great playlists:
Epic ski has a racing section www.epicski.com/f/40/racing-and-big-mountain-competitions but there's also detailed tech-talk on their normal coaching and instruction forum: www.epicski.com/f/9/ski-instruction-coaching.
The NASTAR forum has higher-level technique discussions: forums.nastar.com/.
Here's a great set of articles and videos, mostly on equipment, namely boots and such - great resource for higher-level racing/coaching: www.lous.ca/tech-articles.html.
The newer PugSki forum has a ski school section