Mass Updating Salesforce Profile Permissions

Quick tip: Need to mass object several Salesforce profile object and field permissions?

Use the "Check All" chrome plugin (Link to plugin).

To enable only the Read checkboxes, use the following "Check All In Element" selector:
td[class*="readonlyCol"]

To enable only the Edit checkboxes:
td[class*="displayedCol"]

(Updates: via @dhoechst: there's also this plugin more specifically suited to the task.
April '16: Updated regex syntax for matching columns to reflect recent changes in Summer '16)