Try
Dropbox, it can display lots of different file types (including .txt). You have a folder on your computer that you store files in, that folder is then synced to Dropbox's cloud and you can access them on your phone.
It free (for 2GB of storage) and so is the iPhone app. I use it for a selection of documents I sometimes need to access on the go and it works really well. It takes a while when you first sync stuff (I have about 1.5GB uploaded and it took a few of hours) but after that only parts of documents (or photos, videos etc.) that change are synced.
Or convert them to PDF, drag and drop in to iTunes and use Apples free iBooks app to view them. Its part of the feature set for iOS 4.