There are many file sync apps on the Android Marketplace. Awesome Drop (File Sync) is by far the easiest to use, places zero barriers to entry (read: no sign-up or log-in required), and is available free of charge. It seamlessly transfers any file from your computer to your Droid via drag and drop into any HTML5-compatible browser (Firefox 3.6, Chrome 5 and Safari 5).
Once you install the app and fire it up, you're presented with a red box awaiting a four digit PIN. Head over to http://labs.dashwire.com and you'll see another red box, this time auto-filled with your PIN. Input the sequence into your Droid, and your phone will tell you that it's "Waiting for drop," and your browser presents you with a large button, "Drop files here."
Also, you may use the service to send files to a friend or colleague's phone by passing them a PIN, and then sending them a file.
Drop any file in your computer onto this button, and the transfer begins. If you're on wifi, even big files move from your computer, into the Internet's "cloud," and back to your phone at blazing speeds. My 3G testing was a touch slower, but not any slower than I expected. The more bars you have, the faster it'll go.
After transferring a few files, you'll see them displayed on your phone. Clicking on each will launch the appropriate app to display the file. Clearly, this is a "must install" for all mobile professionals who need to get a file remotely, or take important data with them on the go.
So put away that USB cable. Syncing your must-have files between your computer and your Android device doesn't get easier (or less expensive) than this. As long as the developer's servers are online, you can transfer files anytime, anywhere. (I have yet to experience any "server down" messages.)
So far, users are reporting nothing but solid, fast file transfers, and it's hard to believe Google itself didn't create this app as a standard feature of every Android device.