Community
Nicktech.org - Nick Peronti is one of the best PowerSchool admins in the Chicagoland area. Nicktech.org is his pet project and a great resource.
PowerDataSolutions - A great source for customizations.
PowerSource - Be sure to check out the 'Exchange' where admins share their customizations.
PowerSchool Insider Screencast - Biweekly screencast from Jason S. Great information.
PSUG-IL - The Illinois PowerSchool User Group. Get details on periodic free workshop and the two annual meetings, also free.
PSUG-IL Products/Services - If you are looking for current/former Illinois districts who use a particular product with PowerSchool, this page on PSUG-IL's site is just what you need.
Guides
Database Extensions Advanced User Guide - The official guide from PowerSchool.
PowerSchool HTML Tags - Easily the most comprehensive guide on PowerSchool tags brought to you by Brian Andle of course.
plug-ins & customizations
The following are plug-ins and customizations that I admire from the community. My own work can be found under My Projects.
Multi Select - Select a group of students by copying and pasting a list of IDs, names, or state IDs from an Excel and paste that list into this tool.
sqlReports4 - Design reports and charts on any table in PowerSchool and allow admins and teachers to run the reports while logged into either the admin or teacher portal.
Student Menu Filter - The more you customize PowerSchool, the more menu items you end up with on student screens. This allows you to quickly filter the student navigation menu.
Tools
Blue Steel CPM - A custom version of PowerSchool's custom page manager with a number of enhanced features. Blue Steel is a plug-in but let's call it a development tool since only system administrators will ever see it.
jQuery - write less, do more. jQuery is a fast, small, and feature-rich JavaScript library that is usable in PowerSchool. Just be sure to swap out all '$'s with '$j's