Thanks @OmegaStarScream - CoinSwitch comes close, but you have to specifically select just one exchange manually, so it's not going to give users the kind of multi-exchange instant-best-price settlement possible unless you happen to fluke picking the one exchange that's got the best ask order at the instant your trade is executed (so basically - rare).

@tranthidung - BestChange looks pretty cool - doesn't do trading though - it's just a referral site :-(

Coinigy looks almost exactly what I was after (putting aside the major PITA needed to open accounts at all those exchanges myself, and the big problem of having to fund every account at every exchange for it all to work...): @OmegaStarScream - do you know if Coinigy has a means to *automatically* split my order and settle it across 2 or more exchanges so I get the "top of book" (best) on them both?  The entire idea doesn't seem scaleable to me: if they've got order books from (say) 10 exchanges - that's a lot of data, and if they have (say) just 1000 customers, their outgoing data to just those clients is going to be off-the-charts...  at a guess, I'd say that by the time your order hit any exchange, the top of the book's going to have long-gone ?