I know with hotmail there is an option of creating alias's so that emails will go to a seperate folder.
A feature of gmail I know about but have not used yet is similar to the above but it uses your normal email address followed by a +whatever to create a sort of unique email which also can get put into a separate folder. For example your normal one might be
[email protected] with this you could do
[email protected] and it would still go to your main account but can be easily filtered out. Its a good way of working out what websites are selling your details anyway.