Two scripts one in perl (a very old one, I'm a bit ashamed of it) one in ruby (a bit newer, logic is much better, will not hang on huge routing tables and will not kill the device with too many queries). You could be surprised how much of your network is using the default community strings.
The logic here is first to try to read some standard OIDs with a bunch of different community strings from the "seed" device. If there is a community string that works it will be moved higher in the order in which the script test them. Once the script knows the community string it will query the routing table and ARP table via SNMP to find neighbours and then repeat the whole process on them. The hosts are being distinguished by their local IP addresses, the script generates a MD5 digest from those IPs (gathered also by SNMP) to check if it is a host that was already seen.
Actually the second script was an exercise in learning/trying out ruby. It is not complete. The main function is fully implemented the missing component is a nice way of printing/storing the data.
If you would like to brute force community strings on a specific network range or a single host then I would suggest to use scapy and something like this:
>>> snmpcomm=[comm.strip() for comm in open("wordlist-common-snmp-community-strings.txt").readlines()]
assuming here that 220.127.116.11/24 is your target. I use send() because I just want to send the packets and I don't want to waste time on waiting for replays. In a different session on the same machine I run:
tcpdump -n udp src port 161 and net 18.104.22.168/24
and I wait for the replays. They will include the community string that triggered them.