There are some third-party defragmenters out there, but you really don't need one with a mac. If you are still concerned about it, here's an article about mac's and defragmentation:
Google "download manager for the mac". There are a number of them out there, but I don't use one personally. The only really big downloads I do more than rarely are movies and TV shows over iTunes and I just let it do the downloads in the background.