Espacenet offers free access to more than 70 million patent documents worldwide (70+ countries including Canada and the U.S.), containing information about inventions and technical developments from 1836 to today, updated weekly. Often considered the best place to start a patent search.
See this video for an introduction to searching in Espacenet
The USPTO houses full text for patents issued in the U.S. from 1976 to the present and TIFF images for all patents from 1790 to the present. (You can install a free TIFF reader that is designed in part for reading U.S. patent office files).