MoaD ("Mother of all Databases") is a highly curated subset of the PDB of every high-quality example of protein-ligand binding. It is easily searchable with chemical substructures for ligands.

The IUPHAR/BPS Guide to Pharmacology is an excellent reference website containing details on various classes of drug targets, ligands, and diseases.

Providing researchers with access to millions of scientific documents from journals, books, series, protocols, reference works and proceedings.

DrugCentral has a searchable database of drugs with certain annotated properties from the literature and calculated properties.

Learn how the Library of Congress collects, preserves & provides access to its universal collections. Get the free tools and standards used by libraries, archives, and publishers.

SwissBioisostere is a useful brainstorming tool that suggests isosteric replacements for a given input.

The FDA Orange Book contains information about everything related to drugs that have been patented, including expiration dates on APIs, formulations, etc.

The WIPO patent database is the most comprehensive searchable source for original patents online.

The National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.

Better SAR data means better insights and predictions for your drug pipelines.

The PDB is an incredible trove of highly curated structural information and links to primary references. Whenever I start a new target, I look through as many PDB codes of related protein-inhibitor complexes (if available) as possible to understand what I'm physically dealing with.

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GPCRDB is a database on everything about GPCRs, with tools including sequence alignment and ligand sites

PubChem is maintained by the NIH, and contains comprehensive links on literature, patents, assays, etc. on any given compound.

Bindingdb.org is a highly curated bank of binding data extracted from publications and patents and crystal structure annotations. It's a great place to start to find previously identified inhibitors for a target class.