Non-blocking strobing code?
What you want can be achieved with your MCUs timers and their associated interrupts. Timers are basically counters that run inside your hardware and they run decoupled from your code. Since one usually knows how fast these timers tick (e.g. how long it takes to count from one to two and so on) you can specify events that happen when a certain amount of timer ticks passed.

I suggest you have a look at SimpleTimer and Timer1 (the second page is only for ATmega168/328)

Both pages include some code samples where they specify some method that will be called every x milliseconds.