Listed ECN (entries in bold have confirmed Level 2 visibility)
MBTrading ($400 min balance, level 2, 0.5 pip commission, bad swap rates)
Hotspot FX (not researched)
FXAll (unknown leverage, supports broker and prime broker interfaces as well as trader interfaces)
Currenex (50k min balance)
http://www.flextrade.com/flexfx.php (includes automated trading system, unknown details)
http://www.instinet.com/ (institutional trading API)
interactivebrokers.com
dukascopy
Man Financial
Any more ECNs we should add to the list? Will anyone find more ECN, minimum balance, and if Level 2 quotes are available? I'd like to focus on the protocol and database work (see post ECN Trading )

I'm starting this thread because I think that I'm moving up to ECN based trading. I have had some good experiences with MT4 brokers but they don't offer the news trading or liquidity I'm looking for. Most importantly, I haven't found one that doesn't alter the price. I have an EA that is very profitable on all currencies on multiple timescales, but only on ECN style data. On broker data, odd fluctuations in price make the smaller scale trading unprofitable. The larger scale trades are still profitable, but they happen about 2-3 times per currency per month, and they still have price manipulation SL.

I'm going to use QuickFIX from QuickFixExchange.com and write my own "connect to broker" software. This uses a well defined open source protocol known as FIX, managed by banking committee at http://www.fixprotocol.org/ I won't need to go through the hardest part of the process since the FIX protocol is already implemented by QuickFIX. I have looked at Tradebullet and other systems but they fall short on a few things. Namely, automated trading.

I want the ability to remotely manage 20 accounts running on a remote server, not load up a GUI that requires 20 monthly fees and a ton of mouse clicking. I haven't made a final decision between C# and Python, but QuickFix supports both. I'll likely implement my EA in Python first, and then work with others to port EA over to C# if they prefer. I prefer Python because a .Net 1.1 compiler with IDE is not freely available, and there is more security in Python than C#. (It is easier to pirate C# source code than MQ4. 2 minutes and you have indented source code with variable names. Java is about the same using self-reflection.)

Backtesting won't initially be supported, just running automated trading. It is possible to add backtesting later because I can have a quote server and a client, or just trade against a MySQL database of quote data. I'm interested in backtesting because I want to provide random data backtesting to determine performance. Not the silly bell curve Brownian motion testing that is crap, but randomized data indistinguishable from real data using Mandlebrot Fractals. (I'm studying fractal natures of markets, related to the EA.) I will be able to provide ECN data logging on Bid and Ask into a database, and then export the data for comparison into MetaTrader.

All that said, here are a list of ECN that I'm examining, and one of them claims to be an ECN that supports MT4. http://www.odlsecurities.com/uk/forex_demo.html (Supports MT4, claims to use Currenex)

I have found a few other brokers, and another possible protocol to add later is Trading Technologies but there is a lot of overlap between the brokerages. FIX is more open, TT is more proprietary.

An algo trading thread on FIX: www.fixprotocol.org/discuss/read/2025d30c
A list of FIX exchanges: www.fixprotocol.org/adopters/
The list of exchanges is a partial list, many brokers implement the protocol through partners. I'm aiming at FIX 4.2 support and later implementing FIX 5 support when it is more widely accepted.

My eventual goal would be to to make a simple script that converts EA into C source files and then execute the EA on the server. That interface is still hypothetical, there would need to be a layer of abstraction. (Magic numbers would take some work, etc.) It might even be possible to 'become MT4 compatible' and allow direct trading on the ECN through MT4.

The main obvious advantage for doing this? 0.5-1 pip spreads or trading between spread on over a dozen currencies.