Some other log files are standard on UNIX systems and tend to be associated with specific programs or facilities.
/var/log/wtmp{,x} (the location is not entirely standard) records login and logout events, as well as system reboots. If your system has both wtmp and wtmpx, the latter contains more detailed data. Both are in a binary format and you need to use utilities like last to process them into a human-readable format.
/var/log/lastlog contains information on each user's most recent login and logout, and is also a binary-format log that can be processed with the lastlog utility.
dmesg is a utility that extracts messages buffered by the kernel at boot time and certain critical messages generated by the kernel while it is running. It is mainly of use for investigating kernel configuration issues.
/var/log/faillog (Linux-only?) or /var/log/loginlog are files that record login failure information; these can be of use to investigate user login problems or attempts to break into user accounts.
/var/adm/pacct is the optional process-accounting log; process accounting (if enabled in your kernel configuration and turned on at boot time) records a summary record for each process when it exits, including (part of) its name, the user and group it ran under, the times it started and exited, and resource usage information like CPU time used, I/O statistics, and memory use statistics. Originally this was intended to allow charging for system usage but now it is typically only used for debugging or statistics-gathering purposes. The acctcom utility can process the binary format used by pacct.