Not having my n900 that long, these are the once i use:

• BatteryGraph
• xterm -> root -> mc, awesome filebrowser. i use something simmilalr on my pc (altap salamander). has build in ftp client
• conky
• either diskusage or storageusage, i have both installed
• iptraf
• personalip desktop widget
• ConnectNew desktop widget, great for (dis)enableing wifi connections
• SSH server, to work on my phone from my pc. altap salamander has an SFTP/SCP client build in so it's awesome.
• rootsh, obviously
• systeminfowidget together with load applet, check cpu and mem activity

i also have flashlight installed but haven't used it yet.

obviously i have more installed then this.