FileMagnet (mentioned briefly by Dave in his iPhone apps I actually use post yesterday) is a low-cost wireless file transfer tool for iPhone and iPod Touch. It requires you to install a companion desktop application, also called FileMagnet, which acts as the conduit for file transfer.
Usage is simple enough: drag files you wish to copy into the desktop app's window. Then open FileMagnet on the iPhone, and as long as the two machines are on the same WiFi network, they will find each other over Bonjour, and wirelessly connect. Then your files are smoothly and wirelessly transferred over. Transfer will also happen every time you sync the device, if the desktop app happens to be running.
FileMagnet is a handy place for reference materials, such as Word docs, spreadsheets, PDFs and images, that you're likely to need to carry at all times. It handles entire folders full of files, too.
There are a few problems right now, all of which are addressed on the official FileMagnet FAQ, mostly with a promise that they'll be fixed soon: