For one-off checks or use by non-developers, there is now a web front-end for the valuable epubcheck validation tool.
Uploaded files are run through the validator and any error messages are reported. The error report includes some notes to help decipher the sometimes-cryptic messages (notes are taken from the epubcheck wiki).
epub files are deleted immediately after validation, but take care not to upload any proprietary or sensitive documents. This tool provides no guarantees that any information it receives is secure.
The tool is running epubcheck version 0.9.5. It is planned that the front-end will track updates of the epubcheck library.