While the authoritative storage is Amazon RDS, 99designs is using Redis, MongoDB, and Memcached for transient data:
We log errors and statistics to capped collections in MongoDB, providing us with more insight into our system’s performance. Redis captures per-user information about which features are enabled at any given time; it supports our development stragegy around dark launches, soft launches and incremental feature rollouts.
It’s also worth noting the nice things they say about using Amazon RDS:
An RDS instance configured to use multiple availability zones provides master-master replication, providing crucial redundancy for our DB layer. This feature has already saved our bacon multiple times: the fail over has been smooth enough that by the time we realised anything was wrong, another master was correctly serving requests. Its rolling backups provide a means of disaster recovery. We load-balance reads across multiple slaves as a means of maintaining performance as the load on our database increases.
Original title and link: 99designs: Powered by Amazon RDS, Redis, MongoDB, and Memcached ( ©myNoSQL)