The IP address of users is displayed in a number of places. This is useful, especially - but not only - for blocking spammers.
For purpose of blocking, look into the RecentChanges. Either the IP is visible directly or - if the spammer has entered a UserName - as a hidden HTML which is displayed when you hoover over his username link. Note that this title is only generated (as an additional feature, like other additional functions) when you yourself have entered a UserName.