As a freelance web developer I am always looking for tools that I can use to make my life easier and deliver more value to my clients. WPMU Dev is an excellent resource in my arsenal. They offer a wide variety of themes, and their vast selection of plugins are top-notch. With up-to-the-moment updates I can always be confident that all they offer will be compatible with WordPress' latest versions.
While their product is great, their customer service and community attention is even better. I've dealt with George, David, and several others and have always found them to be very quick to answer (even on the weekends) and super friendly and helpful.
If you design sites, or even just one site, in WordPress, I'd highly recommend checking them out!