EAGLE (Easily Applicable Graphical Layout Editor) is an ECAD program produced by Cadsoft in Germany (American marketing division: Cadsoft USA). It is very commonly used by private electronics enthusiasts, because there is a very usable free demo version for nonprofit use and is available in English and German. Cadsoft has released versions for Microsoft Windows, Linux, and Mac OS X.
EAGLE provides a schematic editor, for designing circuit diagrams and a tightly integrated PCB layout editor, which automatically starts off with all of the components required by the schematic. Components are manually arranged on the board, with the help of coloured lines showing the eventual connections between pins that are required by the schematic, to aid in finding a placement that will allow the most efficient track layout.
It also provides a good autorouter, which once the components have been placed will attempt to automatically find an optimal track layout to make the electrical connections. It does not always manage to find a way of routing all the signals, although it permits manual routing of critical paths such as power and high frequency lines before letting the autorouter handle the other connections.
EAGLE is very popular with hobbyists because the free demo is able to create usable PCBs. (Free ECADs from some companies are crippled so that they won't save or won't print.) The only limitations of boards made with the EAGLE demo are: 2 copper layers; a maximum size of 80mm x 100mm (½ Eurocard)(~3in x ~4in). The demo version of the schematic editor module can only create single-sheet schematics.
The licensing structure for the commercial version is very flexible. In order to use the demo version to make commercial products, it must be licensed for $49 USD. If more is needed, it is also possible to license the required extra modules (schematic capture, layout, autorouter) separately and simply purchase more if needs change.