Quo is an offline slippy mapviewer marketed by Mapyx. As such it competes with MemoryMap, Anquet, etc.
Quo also allows online viewing of OSM Mapnik and OpenCycleMap Slippymap tiles to zoom level 17. Tiles appear to be 1-2 weeks behind the main planet file.
OSM tiles are available for download for offline viewing. The tiles date from 2009 and the highest zoom level is only around level 8.
The free Quo software can be used to view georeferenced bitmaps in TIFF format using WGS84 co-ordinates, but building a full slippy tile set does not seem immediately feasible.