In particular, I would take a look at:

My PoW Work

From that page you can manage your account (make a class, create student logins) and you can use the teacher office functionality to view your students' online submissions and also give them feedback.

 

Calendar

If you have only Current PoW Membership access this page will give you access to all of the problems and accompanying teacher resources from the current season.

 

PoWerful Ideas Newsletters

Read the six features of the current issue and also take a look at the archived issues.

 

Problem Solving Activities

Members at every level (Current, Full, School, District, Trial Account) have access to these resources.

 

Teaching with PoWs

Some of the popular links from that page include:
* The Rubrics
* Think You Don't Have Time to Use the PoWs? [pdf]
* Problems of the Week Articles