This is probably my favorite bra of all time. Not only can you change the straps out and attach them halter-style, racerback, or normal, but you can also change where they attach along the back strap. (very nice for women like me with somewhat narrow slip-off shouders--you can adjust the straps to hook toward the middle, without being crisscrossed, and still hide them under any type of tank top) It comes with a set of clear straps also, though I'll admit those drive me nuts. The last option would be strapless... which makes this the ultimate bra to wear shopping, because no matter what you're trying on, this bra will work with it.
At $50, it's not cheap, but not more than most of Victoria's Secret bras.
Definitely gets the Piper Patter Seal of Affection.