This is a big old list of my preferred WordPress plugins, what I use them for, and why I like them. Hope you enjoy and find it useful!
Tools Jetpack I’m putting this here because Jetpack does so many things. In particular, I like Sharing, Enhanced Distribution, Monitor, and Widget Visibility. A lot of other stuff is nice — like automatic verification of sites for Pinterest, Bing, and Google Webmaster Tools. Enabling Jetpack and this service on websites makes it ridiculously easy to verify these things. Subscriptions are also nice — and when I have multiple authors, Spelling and Grammar always helps!
I really, really like Gravity Forms. It’s powerful and offers a ton of functionality. I’ve developed with Gravity about a year and a half, using it for tons of different things: simple forms, complex forms, credit card payments, user registration.