I like ShipStation. Really wonderful for shipping.
Product Customizer is really helpful for adding lots of variants. I ran out of the ones allowed by Shopify pretty quickly. There is a new one out by the people that do Product Upsell (Shappify) that seems to do the same thing and is about 1/2 the price. Looks good but I haven't tried it yet.
Lexity is a free app. that shows activity in your store. Good stats on when someone is in there, what pages they looked at, time spent, etc.