I have already posted on the wonderful Dropbox as a tool for sharing files between computers (both Windows and Mac). Sadly, at least at the moment, it is not the ideal method for sharing files between a computer and an iPhone. In fact, there is no solution that is ideal for all file types and connection scenarios, which is probably why there are a lot of options available. I have tried quite a few so will share my experience.
There are three aspects to consider. Firstly, is it a file that MUST be stored on the iPhone, say a text document that you want to edit? If not, will you have network access (Wifi or 3G) when you want to use it? Finally, what type of connection is available you are ready to transfer a file.
Dropbox, Box.net, iDisk and other general file sharing programs have iPhone clients. Each has different abilities to view files but none allow you to edit anything from your iPhone. Obviously, you need an internet connection to use one of these services. Of these, I prefer Dropbox because it best integrates with my other computers.
Programs like iThoughts and Documents To Go use files located directly on the iPhone but at the moment, the only common place to store files is the camera roll, so each creates it’s own storage area and hence they have slightly different ways of transfering files on & off. iThoughts can up/download files from Box.Net when on 3G or WiFi, or be accessed directly from a computer on the same LAN; support for Dropbox instead of Box.Net is imminent too. On the other hand, DTG can only sync files with a computer on the same LAN, and DTG must be running on both to do it. However, you can email a DTG file from your phone so you are not completely LAN reliant. DTG’s real selling point is the ability to create and edit Office documents on your iPhone, if you don’t need this, look at something like Air Sharing, which mounts the iPhone as an external device on your laptop. This is my current prefered way of transfering files to my iPhone but before you buy check that it can view the file types that you want.
The ideal situation is one where Over The Air (OTA) syncing happens automatically, in the way that calendar and contacts are. My impression is that most of the file transfer options used today are compromises to work within OS constraints so I hope that version 4, due this summer, will trigger some further moves in this direction. This morning I saw a report on TUAW that MobileMe would become a free service, if so I am sure this would be a big drive towards a standard way of getting on/off the iPhone. With some kind of freemium model Apple would not need to throw away all MobileMe revenue, which makes this a reasonable possibility.