Metatrader can be downloaded at
www.metaquotes.net together with their API.
On their home page is also a list of some brokers that currently make use of the Metatrader platform.
Others using Metaquotes platforms include FXFirst (old application) and InterbankFX (Metatrader).
The API can be called from within programming environments such as C++, VB and Java - but if you don't want to get your hands too dirty then I suggest looking at the scripting environment provided with Metatrader. This allows you to automate your trading to a fairly high degree.