If you have a lot of stuff on your computer that you'd like to have access to at any given time, then you definitely should check out SugarSync. What this app does is it uploads the areas of your computer that you want it to onto it's webserver. From there you can access those files or even send them at your leisure.
The service does cost, but the app is free. That's kind of an odd thing, but with a 45 day free trial, it's not bad for everyone to check it out. If the file that it's hosting is supported by the iPhone, it will give you the option to open it. If it isn't, then it gives you the option to send the file to someone else.
The sending ability is the best part hands down. Where it may take awhile and chug as your iPhone sends something to someone else because of the bandwidth limitations, this app isn't sending it from your iPhone, it's just issuing the command. That means that you press the button and that picture you forgot to send your cousin is now instantly in their inbox.
SugarSync is a really handy app that probably every iPhone user could take advantage of. The issue with it is the cost. It's hard enough justifying paying for MobileMe, it's probably going to be a lot more difficult to actually convince people to pay for this app. Now, if they offered a free version, no matter how small the server space was, it would be a must have app.