During our holidays in summer 2002 I saw this ancient Roman mosaic, which tiles the plane with squares
whose straight sides are replaced by arcs. This is a basic cell to make polyforms a little bit different from
the usual polyominoes.
Putting four or five basic cells together you get 7 or 21 distinct pieces respectively. With the
seven pieces you can easily pack a 7x4 rectangle but to fill a 7x15 rectangle with the 21 penta-pieces will
take you some time. Miro Vicher has this puzzle on his site, too.
For larger constructions a computer program would be helpful.