Because governments regulate the use of chemicals and drugs, they often produce and curate a lot of data about individual substances. The websites below are run by the governments of the European Union, Canada, and the United States. Here you can find information about substances and how each government regulates them.
Drug Information Portal: This website belongs to the US government. Here you can find basic information about most drugs, including generic and proprietary names for them, what they are used for, and their chemical structure. Each record also contains links to research and other information about the drug.
Rx Class: Also from the US government, Rx Class is similar to the Drug Information Portal, but it allows you to browse drugs by various categories (disease, chemical structure, etc).
European Chemical Agency: This website contains similar information to ToxNet, but for the EU. You can find information on individual chemicals and how they are regulated (the information can be a good contrast to ToxNet and other US websites, because the EU tends to regulate chemicals more stringently).
Food and Drug Administration: The FDA website is a good source of information on drug approvals and recalls in the United States. The Drug Approvals and Databases page, particularly, has some useful links.
Environmental Protection Agency: The EPA website contains information on US regulations and EPA enforcement actions.
Government of Canada: Health risks and safety: From this page you can link to information on the regulation of chemicals and consumer products in Canada.
Government of Canada: Drug Database: Similar to the Drug Information Portal, but also contains information on drugs' approved uses and availability in Canada.