1. U.S. Patent & Trademark Office Patent/Application search Old style text-based patent/application searching, which is the most accurate for U.S. patent applications and issued patents.
2. Google Patent Search The best for "smart" text-based searching, and results are shown overlaid on PDF, but remember to review the original publication from the USPTO. Google Patent seems to miss a lot of information and is not as accurate as individual country databases (e.g., the USPTO).
3. European Patent Office Search (espacenet) Find patent applications and patents filed throughout Europe, even those filed in a single country (e.g., Germany only filed outside the Patent Cooperation Treaty).
4. USPTO Public Application Information Retrieval (PAIR) Most U.S. patents have file histories available online - review the "negotiation" between patent examiners and patent applicants to determine what was considered patentable
5. World Intellectual Property Organization Patent Search International patent application and patent search - those applications filed through the Patent Cooperation Treaty are published internationally, sometimes before the U.S. and other countries publish them.
6. USPTO Biotechnology/Chemical/Pharmaceutical Customer Partnership Conferences Better understand how U.S. patent examiners review pending patent applications.
NOTE: individual countries also maintain their own patent application and issued patent databases. The World Intellectual Property Organization provides links to individual countries. For a more thorough search, you will need to access those databases as well as those above.
Several professional search firms are also available to coduct patent landscape searches, such as: