Up until now, you could get unlimited travel all over any German state by regional trains any day of the week (after 9 AM on workdays) with a Laender-Ticket, but you could only travel across all of Germany with a single regional day pass (the Schoenes-Wochenende-Ticket) on the weekends. On weekdays you had to piece together a number of Laender-Tickets, one for each Land you traveled in.
As of August 1, and lasting at least until January 31, you can now have unlimited travel on regional trains anywhere across Germany on M-F after 9 AM and until 3 AM the following day using the new "Quer-Durchs-Land-Ticket" (Q-D-L-T, lit. "Straight across the country ticket").
The Q-D-L-T will be sold online (with €3,50 shipping) or at ticket automats for €34 for the first person, €5 more for each "Mitfahrer" up to four. It will cost €2 more at ticket counters for "personal service".
The offer is only until Jan. 31, but I would hope that it will be made permanent then.
Unlike the Schoenes-Wochenende-Ticket, you have to specify the number of people at purchase time, so you can't start with 2 people and add stranger up to 5 along the way. I would not be surprised to see this feature added to the Schoenes-Wochenende-Ticket and eventually to the Laender tickets as well.