As Windows 98 is a copyrighted product of Microsoft, there is no legal way to download it for free as of 2015. Even though there are several places that offer the operating system for free, none of them are legal.
Because Microsoft never distributed Windows 98 online, it's not possible to purchase a digital copy of the operating system either. Moreover, users buying setup CDs of Windows 98 actually pay for the product keys that come with the CDs. As such, even if you download a free copy of Windows 98, it does not activate without a legitimate product key.
Obtaining software illegally falls under copyright infringement and is a punishable crime. The offender may receive fines up to $93,500 and years of imprisonment.