Patents form a major part of the chemical literature. If the patent you're looking for is in any way chemical or pharmaceutical related, even remotely, SciFinder is by far the best starting point. There are many other search engines and portals that provide free access to patent documents from agencies around the world.
Most national patent office public search sites, while free to use, have poorly designed interfaces, so patience and determination are essential. Pay special attention to the entry format of patent numbers: most databases won't tolerate any variation in the parsing of numbers and country codes. They are mainly useful for locating a patent document you already know about. They are not intended for patentability or prior-art searching, which should be done by professional patent searchers.
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