Petr1868 is Czech user that is mapping with tracer. Tracer Using RUIAN and LPIS. They contains buildings and farmlands. Its easy to draw complicated shapes. Just click and magic happens ;) These travers were developed by Marian Kyral and contributors (sorry i dont know names ;) ) and using servers http://poloha.net where backend od tracer is. Where to click ? Use TMS RUIAN and LPIS layers in JOSM.
Really works !