The 555 timer is great for many applications. When set up properly, the 555 can be used to produce a square wave. With a relatively low component count, it is possible to change this to a Sine Wave. A simple RC filter constructed out of resistors and capacitors is all that is needed. The value of the resistors and capacitors is based on the frequency. This online calculator can assist you in finding the values for a given frequency.
The example below is using 500Hz for the frequency. The resistors are 3.3k and the capacitors are 0.1uF. We can look at each stage of the filter and see the waveform change.
At the final stage you have a relatively clean sign wave.