I've been using Dropbox for storing files online for some time now and I really love it. There's an desktop installer for Windows, MacOS and Linux and it will add a special Dropbox folder to your computer. Any files you put in there will be automatically synched to Dropbox. Install it on another computer and it will download your stored files automagically. Forget about USB flash drives, you'll hardly ever use them again.
By default, all files you upload are private. However, it's very easy to share folders with other users. You can share holiday pictures with selected people very easy. Just share a folder, send a link and by accepting this shared folder all contents are downloaded to your PC or laptop without having to click a lot of individual files.
On my iPad, I use GoodReader to sync PDF documents I want to read, and also photo's. I can upload pictures I take with my HTC Desire Android phone with the Android Dropbox app to my iPad without needing my computer at all. Of course there's also an iPhone app.
With the free version, you'll get 2GB of free space. More than enough for a lot of documents. If you want to sync a lot of photos and/or music, you might want to get a paid version which allows up to 100GB of storage.
Oh, and if you use this link to create a Dropbox account, you'll get me an additional 250MB of storage for free and you also get 250MB of additional storage! Invite more friends and you can get up to 8GB of storage for free.