Game Hippo still commands a high traffic volume, but that is falling every month and getting on Game Hippo seems rather difficult, as nobody seems to be putting a lot of effort into it anymore.
Download.com moves a ton of freeware downloads.
I would also aim for trade CDs (like PC Gamer). They are often interested in freeware. If you take the combined effect of all the CDs worldwide distributed through magazines it is a fair chunk of traffic.
Beyond that... I can't thing of freeware specific outlets that are going to make huge differences.