Donncha O Caoimh (a11n)

Use the cookie left after someone leaves a comment to identify their future visits to your blog.

Blog Voyeur

Log by name where and when users visit your blog.

Ipstenu (Mika Epstein)

JohnnyWhite2007

Spam is one thing, but trolls and sock puppets are another. Sometimes people just decide they’re going to be jerks and create multiple accounts with which to harass your honest users. This plugin helps you fight back by logging the IP address used at the time of creation.
Log into your WP install as an Admin and you can look at their profile or the users table to see what it is. For security purposes a user’s own IP is not displayed to them when they look at their own profile.
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<h3>Privacy Notes</h3>
This plugin adds additional data to a new user’s <code>wp_usermeta</code> data under the <code>signup_ip</code> key. This data is directly tied to the user account, and is only editable via the database. Should a user account be deleted from the site, the data will be automatically deleted.
No external data is transmitted, it all stays on your install.

Register IPs

Does exactly waht it says

When a new user registers, their IP address is logged. Supports multisite and single site!

Blog Voyeur vs Register IPs