If you're trying to connect from one (Leopard) Mac to another, you can use the built-in Screen Sharing functionality; turn the server on from the Sharing System Preferences pane, and either use the network browser (on a LAN) or just open a vnc:// URL.
If you're trying to manage a bunch of Macs, try Apple's Remote Desktop (ARD) software; it's sold in 10- and unlimited-client versions, so if you've got fewer than 5 or so Macs it's probably not worth the money. The client bits for ARD are part of OS X. Screen Sharing and ARD use the same protocol, which includes some Apple-proprietary extensions to VNC which do encryption (either of all data, or of just keystroke/password info) and support adaptive JPEG compression, which gives you decent-enough performance (usable, but nothing like RDP or NX unfortunately).
If you need something cross-platform, check out Timbuktu and TeamViewer (which will punch through firewalls and so forth).