Open Source / free choices for SFlow monitoring?

I've actually been looking at the same type of setup. InMon's sflow toolkit seems to be the choice right now. Unfortunately their download site has been broken all day for me :(. Their tool sflowtool can resend the sflow's as normal netflows. So you could use that to dump it into any of the many very good netflow analyzers.

My personal goal was to get this working to redistribute to netflow-tools so i could use the flowview Cacti Plugin.

If InMon's download site starts working next week i'll let you know how i made out.

ntop also supposedly is able to handle sflows, but i wasn't able to get that working right.

Have you tried using nfdump/nfsen? I have used nfdump/nfsen for netflow v5/v9 and they work without a glitch, really powerful set of tools. I personally have not used them with sflow but I know that there is an option to compile nfdump with sflow support.