Even the most conscientious and clean person among us skips washing his or her feet in the shower every now and then. It can be a pain. It's really important to the health of your feet, though.
Your feet spend most of every day trapped inside a pair of shoes. All the moisture and sweat provides fertile ground for bacteria to grow, and the spaces between your toes are even more hospitable for bacteria and fungi. When you don't wash your feet, you're letting that stuff accumulate from day to day. That leads to unpleasant foot odor -- and eventually to fungal infections and other skin problems.
You don't need a special foot wash to clean your feet. Just make sure you give them a thorough soaping, especially between your toes. After your shower, it's equally important to dry your feet (between the toes again) to prevent trapped moisture from letting all that bacteria hang around. Foot powder can keep them dryer longer, too.