Windows 10 build 10051 leaked this weekend, and with it brought a number of new changes and features over the last public preview build which is 10049. Although build 10051 is just two builds newer, it does include a few welcomed additions.
Spartan in build 10051 received a few new additions, being Internet Explorer can now be easily accessed via the extras menu plus a few other IE integrations. Ontop of that, Spartan also finally includes a Download Manager, something which was missing in build 10049.
Build 10051 is a leaked build of Windows 10, meaning it was not meant to be given to the public for testing. Build 10051 also includes new Mail and Calendar apps, which are universal meaning they run across both PC and phone.