Creating new organization-wide email addresses
You can add additional organization-wide email addresses if desired.
These become available to use as FROM addresses for email alerts, or for any manual emails sent from Salesforce.
You can control who can use the 'from' address by restricting the address to certain profiles.
Creating a new Organization-Wide Email Address
Go to Setup / Admin Setup / Email Administration / Organization-Wide Addresses
Click on the ADD button
You can choose a display name (what the address will appear as in picklists) and the email address you wish to use:
Optionally you can limit the use of this address to users with a specific profile.
Limiting to staff and system administrator makes sense for manual emails -- but if you wish to use them for workflows its best to select "Allow All Profiles to use..." as the workflow may be initiated by someone using the public site or portals.
Save... and you will be sent a verification email to activate this email address and make it available for system use.