SexyTopo is a developing Android application that aims to start where PocketTopo left off. It will run on your phone or a tablet, and is best used in conjunction with a modified Leica Disto X310 (making it a DistoX2 to cavers). SexyTopo is free, but you must buy your own disto, upgrade kit and computing device.
Still has a few unfinished features and rough edges, but is developing rapidly. Not yet ready for 'production surveying'.
Download from Google Play Store
Comparison of SexyTopo with similar applications. Therion In Cave Mapping wiki page.
DistoX, SexyTopo and Therion wiki page.
Just some tentative experimentation in and around Middle Earth so far.