As mentioned before, Frank Lloyd Wright designed several buildings in Oak Park. Two of the most popular sights are the Unity Temple (875 Lake Street) and the Moore-Dugal Residence, which is located near his home and studio.
Frank Lloyd Wright completed the Unity Temple around 1906 after a 1900 fire destroyed the original temple. Wright was a member of the Unity Temple and was asked to design the new temple after the fire on a $40,000 budget. Using concrete and metal reinforcements, Wright designed a modern temple that was practical and went against the Gothic architecture of several of Oak Park's churches. I didn't go inside the temple, but I found the outside of the Unity Temple very imposing and different from many of the churches that I have visited in Europe and the US. The church has services every Sunday and tours daily for about $8. Unity Temple is also gay friendly for members of the gay and lesbian community who want to attend church there.
Another of my favorite FLW buildings in Oak Park was the Moore-Dugal Residence. It is still a private residence, so tours are not available, but one can look at it from the outside and marvel at its half-timbered exterior and beauty. Frank Lloyd Wright designed the Moore-Dugal home in 1895 after he left the Adler & Sullivan firm to strike out on his own. This home was his first independent commission, and it is gorgeous. Unfortunately, an early morning fire destroyed much of the third and fourth floors of this house on December 23, 1922, and Frank Lloyd Wright returned from his work out of town immediately to begin redesigning and reconstructing the home in 1923. So what you see is the reconstructed home and not the original.
I highly recommend taking an independent walking tour of Oak Park to see many of the Victorian, Prairie, and Wright homes in the area. Most of them are private homes, but you can admire them from afar and enjoy their beauty.