GoodReader is a good app too - it connects to most cloud services, plus to other network drives. It's a doc viewer (supports several formats), but also great for managing files. I'm not describing how great it is, I suggest you look at their site to understand all it can do, it's one of those apps that I think might have started as one thing, but continued to grow with features and now does so much. Their page is
here.
Dropbox is for me the most valuable app, already listed, and if you do all the right things you can get 6GB for free - this site explains it
here. Actually you can get up to 16GB, but 6GB you can get by yourself, and since you've got limited storage on your iPad, free cloud storage is invaluable. SugarSync used to be good, but now they charge. Spider Oak is good - you can get 2GB free from them, I use them also you may want to check them out (they're particularly good at privacy).
Evernote - I love this app, can't say enough good about it. Plus, if you want to use your iPad for recipes storage, the sister app Evernote Food is great too. Both are free (though you can subscribe to advanced features in Evernote).
Of course reading apps like Kindle and iBooks, especially Kindle as you can view the books you store in the cloud, download a book, read it, delete it and it'll still be in the cloud.
Hope that helps, 16GB is not a lot of storage, but with some of these apps it is absolutely manageable (and not difficult).