Lib2go is an online program that uses Calibre. I use it at work, with files on my flash drive, when I don't have access to Calibre. It's easier to use than Calibre--because it has no settings involved. Just upload, set limited metadata (which can be glitchy), click "convert," and poof, there's your epub or lrf file. There's no image options, no setting margins, no library management. (And very large files don't work. I couldn't get 10mb PDFs to convert.)
It hadn't occurred to me that the OP might not know about
Calibre, which is definitely much better for regular use.