If its Windows, you can use perfmon to do remote monitoring.
If its a Unix-like system (Linux, BSD, Solaris, etc), there are many ways. Assuming you want it to be agentless (so SNMP is not an option), you will need to have a means of remote access (ssh, telnet, rlogin, etc). Assuming you do, you can open a console window remotely and just run native tools (vmstat, iostat, netstat, top, etc). If you want to monitor many servers at once and store data for later analysis, this method is not very practical and you will want to automate your stat collection. I used to run a bunch of perl scripts that would remotely connect and gather stats at certain intervals and then write the data collected back to text files so I could graph them in a spreadsheet. Or I would hook the scripts into gnuplot so it would graph them as it was collected.
Recently I have been working a on a new tool to do exactly this. I am using it currently, but its still not ready for release (it will be free/open source when its ready). It is written in Java/Swing. You can see some screen shots and info here: http://jmontools.sourceforge.net/
If this is something that might interest you, feel free to email me.