Numerous patent database resources are available for free online.

I've put together a list of just a few of these resources.

Multiple collections
Single collections
Specialized tools
References

Multiple collections

World Intellectual Property Organization (WIPO): PATENTSCOPE
European Patent Office (EPO): Espacenet
German Patent and Trademark Office (DPMA, Germany): DEPATISnet
Patent Lens: Text Search

Single collections

Eurasian Patent Office (EAPO): EAPATIS: Patent search (English)
Eurasian Patent Office (EAPO): EAPATIS: Patent search (Russian)
IP Australia - AusPat: Patent search
Instituto Nacional da Propriedade Industrial (INPI, Brazil): Patent search (basic)
Instituto Nacional da Propriedade Industrial (INPI, Brazil): Patent search (advanced)
Canadian Intellectual Property Office (CIPO, Canada): Patent search
State Intellectual Property Organization of the People's Republic of China (SIPO, China): Patent search (Chinese)
State Intellectual Property Organization of the People's Republic of China (SIPO, China): Patent search (English)
Japan Patent Office (JPO/INPIT, Japan): Patent search - Basic (Japanese)
Japan Patent Office (JPO/INPIT, Japan): Patent search - Advanced (Japanese)
Japan Patent Office (JPO/INPIT, Japan): Intellectual Property Digital Library (IPDL, English)
Intellectual Property Office India: Indian Patent Information Retrieval System (IPAIRS)
Israel Patent Office: Patent search (English)
Israel Patent Office: Patent search (Hebrew)
Office Marocain de la Propriété Industrielle et Commerciale (OMPIC, Morocco): Patent search
Intellectual Property Office of the Philippines (IPOPHL, Philippines): Patent search
Department of Intellectual Property (Thailand): Patent search - Quick Search
United States Patent and Trademark Office (USPTO, USA): Patent Full-Text Databases (PATFT)
United States Patent & Trademark Office (USPTO, USA): PAIR (Legal status)
United States Patent & Trademark Office (USPTO, USA): Assignments on the Web
National Office of Intellectual Property (NOIP, Viet Nam): Industrial Property Digital Library: Intellectual Property Digital Library (IPDL)

Specialized tools

Patent Lens - BLAST Search
Patent Lens - MEGABLAST Search
US Food and Drug Administration (FDA) - Orange Book
NCBI - Retrieve only sequences from patents
NCBI - DDBJ/EMBL/GenBank Feature Table Definition

References

Landon IP - Intellogist
Patent Information User Group (PIUG) - Patent databases
Dutch Patent Information User Group (WON) - Patent database systems