I'm running GitLab (which is a rails app) with postgres (small database, so it's not using a lot of memory) on a $5 droplet. It's using about 50-75% of my memory. Here's what free says:
Depending on your apps' memory usage, you might be able to put two apps on a $5 droplet (I'd recommend adding a swapfile so you don't run out of memory at some point - since they're using SSDs, swapping is not that bad.)
GitLab is using puma (configured to 1 worker/8 threads in my case), and I'm running nginx in front of it. Sidekiq seems to be eating the biggest chunk of my memory (~20%), so I might be able to save some memory by tuning that.
Same as the other reply. I'm using Unicorn behind Nginx. Sidekiq is eating the biggest chunk of my memory too, and I haven't tried tuning it yet. Haven't "needed" to.
