I haven't figured out why I would need this particular IOS feature (IOS documentation says you can at least get a glimpse of what was happening before the logging buffer wrapped over), but it's such a cool one I simply have to mention it ... starting with release 12.2(8)T and 12.3, IOS can tabulate the occurence of each log message. You configure this feature with the logging count global configuration command and inspect its results with the show logging count command:

a1#show logg count
Facility       Message Name      Sev Occur      Last Time
============================================================
SYS            CONFIG_I          5    4 *Jan 16 11:32:57.130
-------------  -------------- ------------------------------
SYS TOTAL                             4

OSPF           ADJCHG            5    2 *Jan 16 11:31:26.434
-------------  -------------- ------------------------------
OSPF TOTAL                            2