Upgrade Your RAM
A memory upgrade goes a long way in improving the performance of your Mac, is easy to do, and inexpensive. Just be sure to research what kind of RAM you need for your particular Mac, and you'll be on your way to a solid speed boost!

Use Compressed Air
Old Macs, especially desktops, collect internal dust easily. Use a can of compressed air to clear this dust, and you're Mac will run cooler, and thereby faster.

Cool Your Computer Down With Fan Control
If you can't access the insides of your computer with a can of compressed air, and you can tell your computer is overheating, be sure to download Fan Control. This freeware app allows you to adjust your fan settings beyond Apple's defaults. It's a must have for an old MacBook or iMac.

Purchase a New Accessory
Instead of purchasing a whole new computer, why not just purchase a new accessory? It will freshen up your desk and make your computer feel a little newer. Try a Magic Mouse or wireless keyboard to make your old iMac feel like a new one.

Upgrade Your Hard Drive
Often, Macs run slower when we run out of disk space. Upgrade your hard drive to solve that problem--in most Macs this is a DIY kind of job.

Start With a Fresh Install
Completely reinstalling the optimal version of OS X for your Mac will go a long way in speeding up your Mac. All the old junk that bogged your computer down will miraculously be gone, and you'll see the difference. Back up truly important files, but don't be afraid to lose a few things along the way.

Clear Out Old Files
If a complete reinstall is a little too extreme and/or time consuming, use a freeware program like Onyx or AppCleaner to delete unnecessary files and completely uninstall unused applications.

Use Your Mac as a Media Server
An old Mac makes for a great media server, whether its streaming files to your Apple TV and PS3 or is hooked directly up to your TV. In the former case, it's basically just functioning as a wireless hard drive.

Stop Programs From Launching at Startup
Mac OS X doesn't come with any programs that launch at startup, but over the years its easy to accumulate a few. Save yourself the wait every time you restart your computer by going to System Preferences > Users & Groups > Login Items to eliminate all the programs that launch when you login.

Buy a New Battery
Nothing is worse than a MacBook with a battery that just won't hold a charge. Instead of buying a whole new computer when this happens, just buy a new battery for around $100.