wtmptail nicely runs in a MultiTail window (in which it can be filtered, merged, and much more).
two tools somewhat related are yawho and whowatch: http://wizard.ae.krakow.pl/~mike/
Changes
1.6: fixed a possible segfault (Linux) 1.4: ported to AIX 1.3: small wtmp files made wtmptail exit with an error. improved man-page.