Things fit into other things perfectly on thingsfittingperfectlyintothings.tumblr.com.
Bratabase is a crowdsourced "bra database" made to help women find the right bra (interview with the creator). According to many studies, over 80% of women aren't wearing the correct bra size: "there's a lot of misinformation about bras out there". For instance, D cups just aren't that big ("Clearly there are a whole lot more D+ cups out there than people assume") and not all D cups are the same size ("from left to right 30D, 32D, 34D, 36D, 38D. Five different band lengths, five different cup volumes. All D Cups"). Most of these links contain photos and therefore may be NSFWish for some. [more inside]