JellyVNC is a simple bookmark manager for the built-in Screen Sharing application in Mac OS X.
It allows you to create local or remote bookmark entries with a username specified.
Choosing the bookmark and clicking Connect will bring up the built-in Screen Sharing application and prompt you for a password which can be saved in the keychain for faster access next time. There is also a bookmark Menulet for quick access from the Finder menubar.
JellyVNC lets you bookmark and use any device which has the VNC protocol running - it isn't limited to Macs.
Once connected via Screen Sharing, you have access to its features including the ability to choose which display to view (on multi-display machines), quality, scaling and full screen.