Half-band filter
A FIR low-pass filter with its cutoff at exactly a quarter of the sample rate, which makes nearly every other coefficient zero for free.
Put the cutoff at fs/4, make the response symmetric around that point, and every other coefficient lands on exactly zero apart from the centre tap. A hundred-tap filter then needs only about half as many multiplications, and coefficient symmetry halves them again. For a given steepness it is about as cheap as a FIR filter gets.
The shape is tailor-made for halving or doubling a sample rate. That is why oversampling in plugins and converters is usually a cascade of half-band stages: 2×, then 2× again for 4×, and so on, each stage allowed to be more relaxed than the last. It is often combined with a polyphase structure so the zeros are never even computed.