I work in a the field, so here is my knowledge about US companies:
Quite a few algorithmic hedge funds are C++ and Linux centric. Examples include Tower Research Capital, Hudson River, Knight, etc. They typically have their infrastructure done in-house, and benefit from C++ speed. C++ is generally more common in investment banks as well. Bloomberg is also C++ based though they don't do their own trading.
However, some serious players are actually in the Java world: D.E. Shaw is the most notable and probably largest. Two Sigma investments are also major players in Java, and then there are a lot of small players.
I'm less familiar with the Chicago and Austin markets (other algo powerhouses) but it still seems to be primarily C++ and Java.
I know that there is at least one firm in NYC that does only OCAML (Jane Street?), and they hire lots of Ph.Ds. who tend to come from a PL/functional background. I have heard stories of .NET shops but they're less common, likely because of licensing costs - Linux is more common in this domain because you can ramp up a lot of firepower quickly.
Finally, quite a few smaller houses buy access to APIs to do algorithms. The company I work for, for instance, provides a trading platform and a Java based API that one can use to implement specific algorithms. A lot of the complexities of programming are hidden so the quants can focus on the algo side.
I would seriously advise against writing your own trading platform. The market is saturated, and you usually have to hand optimize for years to get good performance. If you have an idea for a clever algorithm, you may be better off licensing a trading platform or joining an actual fund.