Fonts used for the splash screen and web banner: Source Sans Pro 

For specific font sizes, see the corresponding Gimp project files on Google Drive (QGIS_Graphics folder)
Splash material: https://drive.google.com/drive/folders/0Bwc-5JFVTnfIMUwyLTU2cjI4MEU?usp=sharing)
Web banner material: https://drive.google.com/drive/folders/0Bwc-5JFVTnfIYWUzMV9HVjJNRTg?usp=sharing