The USGS recently announced that they will be reprocessing the entire Landsat image archive. Scene will be precision terrain processed and radiometrically calibrated across the family of Landsat sensors. These images will be distributed on the USGS Earth Explorer site and identified as Collection 1 data. The highest quality scenes will be Tier 1 data and other scenes will be released as Tier 2 data.
Landsat 4-5 TM and 7 ETM+ data will begin to arrive on Earth Explorer in late August and Landsat 8 OLI data in December of this year. There will also be higher level products released based on the new Collection that includes Surface Reflection and several spectral indices.
See the YCEO FAQ and the USGS Landsat Collections page for more information.