There can be lots of examples for this.
Some specialized leaves are those of insectivorous plants - sundews, Venus flytraps, pitcher plants, and butterworts which are adapted to trapping insects. Another example would be petals of a flower or bracts of a poinsettia (which people mistake for the petals) which are modified from leaves. The bulb of an onion is made of modified leaf stems. Spines are modified leaves as well.
Some specialized stems for storing food for a plant are tubers and corms. Prickles (like a rose has) are modifications of the stem.
Some other examples: http://www.cliffsnotes.com/WileyCDA/Clif…