Camelbak Racebak - useful for 50 and 100 mile TTs, but the water does tend to heat up so you use that first and then reach for the bottle in the latter stages. For some riders it is more beneficial due to their position, but as maurice said it can also be a problem against the pointy helmet.
The hydration methods seen in the pro peloton have been varied - some have used bottles behind the seat, others standard bottle on frame, and others (notably Garmin) with the Racebak. Each probably has its benefits but they'll be rider specific.