Where can I find real-time flight tracking with the least amount of delay?
You can buy a DVB-TV USB stick for around €20, which will allow you to receive ADS-B data:



Up from that, you can get a high-end ADS-B receiver that costs about €750:



Finally, FlightRadar24 offers free hardware in selected locations:



Bear in mind that these devices will only allow you to receive signals within a relatively small area.

One thing worth noting is by contributing the data received by these devices, you get a free FlightRadar24 Premium subscription.

Lots more information is available at FlightRadar24 > Increase Coverage.