A patent for an invention is the grant of a property right to the inventor, issued by the United States Patent and Trademark Office. This is a good background on patents and what the USPT does.
Google Patents covers the entire collection of granted patents and published patent applications from the USPTO, EPO, and WIPO. US patent documents date back to 1790, EPO and WIPO to 1978. You can download PDFs of US patents by clicking on the “Download PDF” button near the top right of a patent page.
Some patents are more important than others, and some are really fundemental to our modern life. Here is a list of some of the best from the folks over at Popular Mechanics.