Mining algorithms that have aspects that are thought to be harder/more expensive to implement as ASIC chips.

For example, Ethereum implements a number of tactics to achieve that:

- relatively large memory size and bandwidth requirements.

- linearly growing memory requirements over time.

- announced plans to deprecate mining altogether and switch to proof-of-stake, turning all specialized hardware into pumpkins when it happens.

I don't know that it's necessarily enough to make ASIC miners impossible, but it seems to have had some success at keeping the bulk of the mining on GPUs, longer than some of its peers.

I'm only tangentially familiar with it so someone else please correct me if I'm wrong.

Etherium requires a significant memory footprint to compute, whereas BTC and LTC do not. As such, an ASIC was able to have a heap of compute power but very little memory and it'd crunch through as much hashing as a GPU with far less upfront cost.

Etherium on the other hand requires lots of memory, so to add on the memory to the ASIC you essentially kill its competitiveness against a mass-produced GPU from a capital investment point of view.

Bitcoin uses a hashing algorithm called SHA256. This algorithm is easily implemented in hardware and is very parallelizeable. It's a simple pipeline with some adds, rotations, bit shifts, xors, etc. Once people started to build ASICs to mine Bitcoin, GPU mining simply wasn't competetive.

There are many other types of hashing algorithms, many of which aren't as easily pipeline-able. For instance, if a hashing algorithm requires a few megabytes of memory, than making a custom ASIC isn't going to have such a slam-dunk advantage over a GPU.

Some hash functions (eg. scrypt) are designed to make ASIC implementations cost-prohibitive, such as by requiring enough memory that off-chip DRAM working memory is needed. When the memory interface is the bottleneck, an ASIC is not going to offer substantial advantages over GPUs.

The algorithm used by the coin is prohibitively expensive or provides little to no increase in performance compared to GPUs.