Two databases are commercial, Derwent and SciFinder/CAS. The rest are free.
Derwent Innovations Index, is part of the Web of Science family. It is a commercial database that your university library usually subscribes to. Students at Copenhagen University can access Derwent Innovation Index . The link works directly from campus computers and requires a login and password when off-site. Similarly, students from the Technical University, DTU, may access Derwent by logging in via this link. Students from Copenhagen Business School (CBS) access Web of Science here.
Derwent offers a very flexible system of hierachical database queries. Derwent furthermore neutralizes the inventors attempts to make their application non-searchable. Derwent does so by replacing the original abstract with their own using suitable keywords.
USPTO is the patent authority of United States. Their Patent Center offers access to patent and application full text databases. Their advanced search interface supports hierarchical searches and the entire patent text may be queried. US applications have only been published since 2001. (Patent Center is still new and suffers from some browser compatibility problems. Be prepared to experiment with different web-browsers).
National Center for Biotechnology Information, NCBI, provides a BLAST service against sequence listings in patents. Select type of BLAST (aminoacid, nucleotide) and on the next page, choose patented sequences as the database to search (default is nr, the non-redundant database of all organisms).
EMBL-EBI provides a tool, Protein similarity Search, which can also do sequence searches in patent databases.
SciFinder is a research discovery tool that allows you to search the CAS databases for patents on the basis of chemical structures. Text based searches and sequence search are also supported. SciFinder is a commercial database that your university library usually subscribes to. Students at Copenhagen University should create a personal account here, and then access SciFinder here. Likewise, students at DTU create an account here, and login here
CAS monitors, indexes, and abstracts the world's chemistry-related literature and patents comprising such fields as biomedical sciences, chemistry, engineering, materials science, agricultural science etc. Here is the entrypoint for SciFinder tutorials.
CIPO, the Canadian patent authorities
IPAustralia, the Australian patent authorities
PVS-Online, the Danish patent authorities
PRV, the Swedish patent authorities