Valve has included a defragmenter in the Steam client. Each game installed can be selected and right clicked to bring up the properties box. Once in the Properties box, select the Local Files tab and click Defragment Cache Files to begin defragmenting the game.
The author's blog has some screenshots of the process.
[crarko adds: I haven't tested this one. Here and here are some good discussions on the 'Hot-File-Adaptive-Clustering' method Apple uses in HFS+, and its effectiveness. On the whole it's been quite good. The 20 MB file size limit mentioned is one of the limitations, along with the number of extents used. I don't know if you still need to be wary of defragging a FileVault home folder, but I'd err on the side of caution and make sure I had a known good backup before trying it.]

