To cut boards and other things in straight lines, a circular saw is just the thing. The saw has a voltage of 18 volts and would surely make a good addition to your toolbox.
Conrad Electronic
Metabo SET KS 18 LTX 57 Cordless handheld circular saw Cutting depth (max.) (90°) 57 mm w/o battery, incl. case, incl. folding rail, incl. stopper 18 V