The American Institute of Physics has a summary page, part of a series of pages on the subject. They are authored by the same scientist and science historian, Dr. Spenser R. Weart who wrote the book on Global Warming described and pictured at the top of this page. The pages brings you up to date from the 2003 book, with keylinks to other websites where the details can be found.
The amount of evidence is staggering!
For those so inclined to do their own analyses, there are tons of data available from monitoring websites around the world. One of the favorite data sources is the Hadley Meteorology Centre, a British Government facility, in the UK. Their graph of the global annual average near surface temperature since 1891 is based on the monthly average and annual average data. They also publish the actual data in tabular format that can be easily imported into almost any spreadsheet like MS Excel, Lotus 1-2-3, Open Office or Star Office Calc, then plotted and analyzed seperately from ones generated by the Hadley folks.
Is the data from which the averages are calculated valid? There's little doubt mostly because there has been such widespread interest in the subject and it has been the attention of so many capable people.
Are the answer and the calculations simple? In a word, NO! They are complex and require attention to all sorts of details including all the other influencing factors involved in cyclic and irregular changes in the Earth's temperature and its distribution.
The recent, popular book, A Matter of Degrees by Prof. Gino Segre at the University of Pennsylvania talks about aspects of Earth's temperature history and the many astronomical and other physical factors that influence it.