https://earthengine.google.org/, currently in beta testing. Looks like a promising project that can combine multiple (free) raster data sets and provide a set of processing tools (mainly classifiers).
There is also JavaScript and Python API: https://code.google.com/p/earthengine-api/ (available only for a limited group of testers).
Hydrology-related data:
- HydroSHEDS, Drainage Directions, 15-arc second
- HydroSHEDS, flow accumulation, 15-arc second
- HydroSHEDS, hydrologically conditioned DEM, 15-arc second
- ... ?
