Are there any (not a resource hog) solutions to have statistics about the traffic on an OpenWrt router? :O e.g.: traffic per IP address, monthly statistics, etc. :O
Traffic counter on openwrt?
openwrt
To monitor traffic and other information about my OpenWrt router I use munin.  I detail my experience in my posting on Monitoring with Munin.  It doesn't specifically address traffic by IP address, which a tool like ntop might be better suited for.  The firewall can be configured to gather such accounting data, but you would need to extract and summarize it.  I use shorewall-lite to build my firewall rules.  I haven't posted on my experiences yet.