These sites offer most volunteer/exchange/house sitting opportunities in Portugal:

• helpx.net
• workaway.info
• wwoof.pt
• worldpackers.com

You may also find some here:

• barterweek.com
• gooverseas.com
• helpingabroad.org
• hosteltraveljobs.com
• mindmyhouse.com
• portugalist.com
• staydu.com
• trustedhousesitters.com
• voor-elkaar.net
• workingabroad.com
• yogatrade.com