RedSky Financial
Offers trading in stocks, bonds,FX, futures and options via fix or proprietary C++ API. A datafeed also available via a C++ API.

FIMAT
Provides a fullrange of electronic trading services and support for a wide range of 3rd party software providers.

MAREX
Options & futures trading via a wide range of 3rd party software providers.

Interactive Brokers
Global coverage of Futures, stocks, forex and bonds with free API. Trading via FIX is also avaialable for high volume traders.

Lime Brokerage
Low latency US stock trading via API

Genesis Securities
US stock trading via API

eFloorTrade
Global futures trading with API interface via dll to the eFloorTrader platform.

Global Trader
Direct access CFD trading of global stock markets via API.

ECN Trading
Provides a n ECN trading service via API or FIX

TerraNova Online
Trading of US futures, options, stocks and FX with API support

MB Trading
Support US markets with API and FIX interface

Oanda
FX trading via API

Gain Capital
Offers free API access to it's FX trading facilities.

Strategy Runner
Hosted automated futures trading using a proprietary software platform.