Also, not all Mac users may need access to log-in items installed by every application. You can speed up things for other users by removing log-in items that they don't need from their user accounts.
To stop apps from starting up when you log in, go to the Accounts pane in System Preferences and click the Login Items tab. Then select the log-in item you want to remove and click the minus button beneath the list of log-in items. (If the Hide box next to a log-in item is checked, it will still launch at log-in, but you will not see any indication of it launching; this is typically the case for helper applications or processes, such as the iTunesHelper application.)
If you later discover that removing a log-in item creates problems with an application or component, launching the item manually often resolves the issue, and some helper processes will automatically launch when their associated applications are started. To restore a log-in item permanently, you can locate the file on your hard drive and drag it back into the list box, or use the plus button to add it through a file open dialog. This is the same approach you would use to add additional log-in items.
7. Do without the 3-D effects and animation
Leopard's interface sports many 3-D and animated effects. From the 3D Dock with its Stacks icons, to the optionally translucent menu bar, to a number of Finder animations that were originally introduced in Tiger, Leopard is chock full of high-end graphics goodness. Of course, those fancy graphics require rendering power. This can make Leopard seem sluggish on some older Mac models.
One solution to this problem is to disable some or all of these effects. You can revert to Tiger's 2D Dock using tools such as TinkerTool (free) or LeoColorBar (free), turn off the translucent menu bar using the Desktop & Screensaver pane in System Preferences, and display folders in the Dock as folders rather than Stacks (control-click on each folder in the Dock and choose Display as Folder). You can also disable animation effects in the Finder (as well as adjust several other system variables) using TinkerTool.
8. Remove the fonts you don't use
Getting rid of unused fonts provides both a performance and disk space benefit. Obviously, every font file is a file that takes up space. However, Mac OS X must read the installed fonts as part of the start-up and log-in process, making a large number of fonts a performance issue.