Slow App Launch

I noticed that recently that all decent sized apps launched slower than before. I got the spinning color wheel for a while before I can use the app. The spinning beach ball also reveals itself when switching modes within certain apps. Might this be due to my hard drive became much fragmented? My OS is latest version of Snow Leopard. All the posts on the Internet says that with modern OS X, I would not need to worry about defragmenting the hard drive. Do I have an option of defragmenting my hard drive if I wanted to? If so, what do you recommend me to use? Thanks in advance!