GuideStar – Gives information on nonprofits and Form 990s. The Business Library has a subscription to GuideStar Premier which gives more detailed information. Must be used in BIC.
WealthEngine – Gives information on nonprofits.
Idealist.org – Really a job board, but by clicking on Organizations you can modify search to find specific organization.
Charity Navigator – Register as a FREE user and look up charity. Gives very good data if specific charity is listed.
JustGive – Aimed at helping people donate wisely. Look at Nonprofit Resources or search with charily name.
Even though registration is required for all data, there is a substantial amount of information given without registering.
BoardSource – Requires membership but can get some valuable information by going to Resources and Q&A.
National Council of Nonprofits – Does require membership but you can get some information without joining.