Reuters
Data aggreagator with comprehensive global coverage

Comstock
Data aggregator with broad-band feed capabilities and extremely comprehensive global market coverage.

Fixnetics
Low latency wide bandwith feeds.

QuantFeed
Low latency data feeds via leased line or internet.

IQFeed
A very cost effective data-feed provided by DTN. Covers most US markets and major European futures exchanges,

TickPlusData
Relatively new historical data provider offering a range of European and US, stock futures, index and currency data. Provides full level 1 tick history.

TickData Inc.
Tick history provider offering many years worth of tick history for US stocks and a range of global futures.

Olsen Data
Tick and quote history on a wide range of US and EU markets going back 15 years. A very comprehensive set of forex data is available.