GuideStar - gathers and disseminates information about every single IRS-registered nonprofit organization.
ProPublica's 990 Database - Use this database to search over 1.8 million tax returns from tax-exempt organizations. Browse raw IRS data released since 2013 and access over 8.3 million tax filing documents going back as far as 2001.
The NonProft Times - A leading business publication for nonprofit management. The NonProfit Times delivers news, business information and original research on the daily operations of tax-exempt organizations.
Insights Nonprofit page by Stanford Business School. Keep current on trends and issues.