I'm looking to get my hands on an API (hopefully one that's platform agnostic) that allows me to retrieve real time (or as close to it) equity prices / volume (hopefully).

I'm not a professional trader and can't afford anything really expensive (although I'm willing to pay a fair bit a month for such a service). I'm seeking to monitor about 500 equities (SP500) to start.

Does anybody have any suggestion? I don't want to use something like Google, because I would in effect be stealing a lot of bandwidth and could be blocked for abusing the service.

+2  A: 

Google is a relatively shockingly developer friendly company. I have had some interactions with them about volumes of API use. You can hit them quite a bit without getting blacklisted.

One project that I am aware of being developed by Collaborative Software Initiative: FeedHandler:

I am not sure what the cost of the feed would be like. They would be able to track all of the stocks.