Norton periodically tests sites for malicious content. If none is found during the test, the site is rated safe. It is important to remember that a "safe" rating means only that there were no threats found at the time a site was tested. Even the "safest" of sites could be compromised at any time, which is why Norton has many additional layers of protection against online threats. For that reason, you should consider a "safe" rating as a guideline based on past testing of a site, and not a guarantee of current safety. The main value of site ratings is in finding and alerting you to confirmed malicious sites, which you can then know to avoid.
https://safeweb.norton.com/about
https://safeweb.norton.com/report/show?url=sourceforge.net
Note: SafeWeb offers historical reputation rating.
URL: sourceforge.net/
Detection ratio: 0 / 68
Analysis date: 2016-09-25
> http://betanews.com/2016/02/11/sourceforge-pledges-to-clean-up-its-downloader-act/
> http://www.howtogeek.com/254042/the-freeware-download-sites-that-dont-force-crapware-on-you
YMMV