The following graph shows the global temperature development during the last 130 years (Hansen et al., 2006)
This data is from resarchers of the Goddard Institute for Space Studies (GISS), but there are also groups from the University of East Anglia and the National Climatic Data Center working on similar data. They are based on the same observations in the end, but they use different methods for analysis and thereby provide some cross validation of the methods.
The data this graph is based on comes from meteorological stations on land (Hansen et al., 2001), satellite data and ship-based data on the oceans.
One difficulty in analysis the data from stations is the influence of urban development, cities produce heat which could bias the results. To correct for that, Hansen et al. used satellite light data to adjust stations affected by urban development (Hansen et al., 2001).
There are not many meteorological stations in the arctic, for that region the data is based primarily on satellite measurements (Comiso, 2006).
Independent from the temperature data there are also other indicators for a global warming trend:
The conclusion that global warming is
a real climate change, not an artifact
due to measurements in urban areas, is
confirmed by surface temperature
change inferred from borehole
temperature profiles at remote
locations, the rate of retreat of
alpine glaciers around the world, and
progressively earlier breakup of ice
on rivers and lakes
-- Hansen et al., 2006
All the scientific articles I linked are freely available, if you want to dig deeper into the details.