I built custom storage for Twitter back in the 2010-12 period. There wasn't much off the shelf in those days that worked out-of-the-box at scale besides Cassandra, and Twitter had a well-documented attempt at using Cassandra for primary workloads that failed due to the amount of variance in IO and latency for high-volume workloads.

Most of the time, we were building custom distribution layers on top of open source storage (e.g. memcached/mysql/redis/etc). I think blobstore was the first thing twitter put in production that was mostly custom, followed a year or two later by manhattan. I'm not sure if there's even now good open-source competitors for those projects, largely because any reasonable smaller company uses s3 or dynamo.

There's plenty of open-source things twitter created, or nurtured out of the existing ecosystem, from mesos to memcached to some of the hadoop/scalding/parquet stuff.