I use two different tools to monitor my servers. The first is Cacti/RRDTool, which tells me how the network is doing. The second is Munin, which tells me how the particular servers are running (CPU, memory use, disk, etc). Both of these tools are free.
If you are just looking to monitor a single server over a screen sharing connection, then you could use my system monitoring app, XRG. Just set the refresh time to update once every minute, and you can see a pretty long history on the graph whenever you login (assuming you can leave an account logged in 24/7).