The Centre de Données astronomiques de Strasbourg offers a number of useful tools, the most prominent of which being Simbad, VizieR and Aladin. These all allow you to crossmatch your sources of interest (and even your image data, in the case of Aladin) with numerous catalogues in their database, an invaluable means of characterising the objects you are studying.
Simbad is a database of astronomical objects from stars to nebulae to galaxies. You can search single coordinates or upload a whole list and then search for matches within a given radius. See the database in action: simbad.u-strasbg.fr/simbad/
VizieR is a catalogue service, containing thousands of catalogues from various papers over the last 50-100 years including up to date and recently published databases. It is one of the best places to get the catalogues in a universal and complete format, with a lot of customisability in the output of any searches or downloads. You can access the catalogues at: vizier.u-strasbg.fr/
Aladin is a sky atlas, allowing you to load in your own data (in FITS format) and then use crossmatching tools like Simbad and NED in a visual way. It's a great way of exploring multiwavelength data and catalogues, with lots of built-in VO tools. A recent addition in version 7 includes the all sky maps section, which lets you download and view available all sky maps in various wavelengths. Extremely useful! Download or use it at: aladin.u-strasbg.fr/aladin.gml