Elann gives 4 or 5 good-sized, color-coordinated samples of in-house and discontinued higher-end yarns that will be going on sale for that month. I don't know what it is, they're like candy to me! I've been a member for about 3 years. Color snips are also included, and I rely on them more than I expected to. I appreciate the color-coordinated samples for scrumbling and combining swatches. For designing, I can't use the discontinued yarns but Elann's own yarn line is growing. I plan to point out the merits of 3 of their yarns in the near future.
The Yarn of the Month Club is fantastic for designing because all yarns are current. I've referred to these samples often when swatching up design proposals. Often I'll see a new yarn announced and wonder when it's ever going to show up at my local yarn shop, and then a sample of it shows up in the YOTM mailing. Every month 4 samples and a free pattern arrive (they're almost all for knitters though) plus every other month a truly informative newsletter, Knit Dish, is included. I've been a member for about 2 years. You can hear a short interview with Sara Lucas at Craftsanity.
The Freeform Crochet Club is new to me: I've only received the first month's installment and have not yet been able to log in successfully to read the newsletter. Laurie Wheeler is not only a hand spinner, she creates yarns with crocheters specially in mind. So how can I not join? When I opened the package, I smelled the lanolin before I even saw the yarns! A handy dandy laminated info sheet was included. I'm looking forward to finally meeting Laurie at the Oakland CGOA conference this month. I hope to get her talking about yarn twist.