If you don’t already know about WPMUDEV.org I suggest you check them out, they have a lot of free plugins and such that are better than many “premium” offerings!
Basically, WPMUDEV is a place for the more hardcore WP users. Those who want to have full access to not only a huge array of plugins, themes, etc. but to world-class support along with them. Plus they’ll do custom development work in certain cases, have a pretty active community, etc. They also tend to be pushing the envelope in terms of plugin and theme design, and I know personally I’m taking a lot of ideas from them that I’ll be incorporating into my own products.
Anyway, the site is a bit on the pricey side if you just want a plugin or a theme, but if you truly will make use of all their resources, the value is there. But if you’re like me and like to get deals & discounts wherever you can, then today’s your lucky day! Appsumo is running a special deal where instead of the normal $420/year membership cost, it’s just $125/year (might be gone by the time you read this). Frankly it would be hard to not get $125 worth of value out of WPMUDEV in a year, so to me this was a pretty simple choice.
I wouldn’t recommend this for the more casual WP user, but if you run a lot of sites (especially if you use Multisite!) or are looking to do more sophisticated WP marketing, I say take a good look at WPMUDEV!