A flashing BELL LED taillight from Wal-Mart for $8 or $9 together with a $7 LED headlight from OBI in Germany. They make it possible for me to ride during hours of darkness. Without them my riding time would be very restricted.

Front and rear Suntour derailleurs in good condition, both for a total of $2, from a bike in a scrap yard to replace poor quality OEM derailleurs on my 10-speed bike.