Primarily, this depends on what you've got on your machine. There's another FAQ item here that can help you identify where your space is being absorbed, but here are some ways to actually free up space:

For identifying drive usage, try WhatSize:

macupdate.com/info.php/id/13006

WhatSize is basically a GUI front-end for the du UNIX command.

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Another place you can free up space is by removing extra printer drivers that are not used:

www.macupdate.com/info.php/id/8070

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Languages absorb a LOT of space and OS X defaults to installing all of them. Here are some apps that can strip extra languages out:

Monolingual will remove extra languages from your applications:

www.macupdate.com/info.php/id/7758

TinkerTool System will remove extra languages from the operating system:

www.macupdate.com/info.php/id/13662

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Youpi Optimizer can also remove extra languages:

www.macupdate.com/info.php/id/6934