According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software. XMule for Mac lies within Internet & Network Tools, more precisely General.
From the developer:
xMule is a client for the eDonkey peer-to-peer file sharing network. xMule forked from the LMule client, which itself was originally a fork of the eMule client, intended to bring it to the Linux platform.
You may want to check out more software for Mac, such as aMule, which might be similar to xMule.