Reduces the memory usage in the checkout process when using Ebizmarkets_MageMonkey (will be included in future releases).

README.md

Open app/code/community/Ebizmarts/MageMonkey/Helper/Data.php and search for

$last_order = Mage::getResourceModel('sales/order_collection')
    ->addFieldToFilter('customer_email', $customer->getEmail())
    ->addFieldToFilter('state', array('in' => Mage::getSingleton('sales/order_config')->getVisibleOnFrontStates()))
    ->setOrder('created_at', 'desc')
    ->getFirstItem(); 

and replace it with

$last_order = Mage::getResourceModel('sales/order_collection')
    ->addFieldToFilter('customer_email', $customer->getEmail())
    ->addFieldToFilter('state', array('in' => Mage::getSingleton('sales/order_config')->getVisibleOnFrontStates()))
    ->setOrder('created_at', 'desc')
    ->setPageSize(1)
    ->getFirstItem(); 

In our case the memory footprint at this code line was reduced from > 1 GB to about 17 MB.

See Conquer the 5 Most Common Magento Coding Issues to Optimize Your Site for Performance, page 14 for the background.