How can I use the USB block erupters to mine Namecoin? Since the block erupter is useless to mine for Bitcoin now. From what I understand the only other two options would be is to mine Namecoin or Devcoin since they use SHA256 encryption. But how can I mine these? Would cgminer work? or is there another app that i need to use? 

Also are Namecoin and Devcoin the only ones that use SHA256?
In theory, you can mine any altcoin that uses double-SHA256 hashing algorithm as a proof of work with any ASIC device. The process should be similar to mining Bitcoin - you would most likely want to connect to a mining pool or solo mine in a configuration analogous to mining Bitcoin.

Also note that Namecoin supports merged mining, meaning that if you mine for a Bitcoin block through a pool that supports merged mining, your proof of work can still be used to create Namecoin blocks. This effectively makes you mine for both currencies with the same amount of hashing power, increasing your expected return rate by the amount of other coin mined (so instead of mining just Namecoin, you would be getting some extra BTC kickback as well for your submitted shares).