Aladin is a Java-based tool which allows exploration of digital space images or full sky surveys, can superimpose entries from astronomical catalogues or databases, and access related data from Simbad, VizieR and other public archives.
The program is aimed very much at the astronomy enthusiast, but don't let that put you off. It's also interesting to anyone who likes to explore space imagery.
As long as you have Java, there's no setup required. Aladin is a single executable, and you can just download it and run.
If you know what you're doing, enter an object or coordinates in the Location box to start viewing.
If you're a newbie, click the DSS or SDSS buttons beneath the Location box to load a particular survey-- a complete set of astronomical imagery.
Viewing is much like any other 3D navigator you've ever used. You start by looking at a 3D sphere; click, hold and drag to move the sphere in 3D space; when you see an interesting area, move it to the centre of the screen and spin the mouse wheel to zoom in.
Images may be copied to the clipboard (Ctrl+C) or saved (File > Save...) as usual.
To begin finding out more, click File > Load Catalog > Simbad > Submit, and the image will be overlaid with information about the individual objects you're viewing.
There's no space here to discuss even a fraction of what the program can do, so to give you some idea, here's the developer's "Main features" list: Zoom; Pan; Rotate; Overlays; Multi-views; Many projections (Sin, Tan, Aitoff, Mollweide, etc); Any coordinate systems (FK4, FK5, ICRS, GAL, SGAL, ECL); No image size limit; Million source overlays, Most of astronomical formats (images: HiPS, FITS, PDS, HEALPix map, JPEG, PNG; cubes (HiPS, FITS); tables: HiPS, FITS, VOTable, S-extractor, IPAC TBL, ASCII, etc; regions: XML, STC, DS9, IDL); Powerful tool boxes (images: color map, contours, crop, re-encode, color composition, pixel computation, resampling, astrometrical calibration, mosaic, photometric measurements, etc; Catalogs: filter, split, merge, x-match, scatter plot, etc); VO standards: JSAMP, CS, SSA, SIA, etc; Extendable: plugins, VOApp interface; Fully scriptable.