Galorin,
I'm sure that there's a lot of purpose-built stuff for this, but personally:
What do you mean by "tracking the systems"? Monitoring their status? Tracking config/log files? Tracking changes?
For monitoring:
NMS - Nagios, Zenoss, Hyperic, etc.
Log files - Splunk (I don't use it), Zenoss, a few others
Config files - ticketing system, RCS/CVS by hand, Zenoss, Hyperic, internal wiki.
Change Tracking - ticketing system and wiki
Scheduling/Planning routine, recurring tasks:
My normal calendar (at the moment, KPilot/my Treo, but in the future a good iCal implementation)
Scheduling/Planning non-routine or non-recurring tasks:
Ticketing System (I use Eventum, but there's a bunch, RT, etc.)
In looking for advice/answers to something like this, you should consider the mailing lists for
SAGE (the USENIX System Administration SIG) and
LOPSA - the League of Professional System Administrators. Both have lots of knowledgeable people who are happy to provide advice.
Also, as a general reference, Tom Limoncelli's
The Practice of System and Network Administration is an invaluable reference.