Great Cheap (or Free) Mac Software
Below are some examples of relatively inexpensive or free Mac software that I personally find of great value and use on a regular basis. I have no affiliation with any of these software vendors and my recommendations are based solely on my firsthand experience.
Boinx Mouseposé
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Mouseposé 3 is an essential tool for everyone doing presentations or demos. If turned on, it dims the screen and puts a spotlight on the area around the mouse pointer, easily guiding the audiences attention to an area of interest.
Mouseposé runs on OS X 10.5.8 or later and is available for $4.99 from the Mac App Store. For more info, go to Boinx Software at: http://boinx.com/mousepose/

Overflow
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Overflow is an application designed to quickly launch applications, open documents, or access folders while reducing the number of items needed in your Dock. Anything you want can be added to the Overflow interface, making it accessible through a few simple mouse clicks or keystrokes. The interface is resizable, and fully customizable. Create separate categories for your applications, work files, games, or anything else you want to be able to access quickly.
Overflow requires OS 10.4 and is available for $14.95 from Stunt Software at: http://stuntsoftware.com/Overflow/

Default Folder
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Save your files effortlessly and open your folders instantly—Default Folder X makes Mac OS X's Open and Save dialogs work as quickly as you do. Custom keyboard shortcuts put your favorite and recent folders at your fingertips. Pop-up menus let you navigate your folders and open Finder windows. Open, save, and get back to work: Default Folder X is workflow for the rest of us.
Default Folder requires OS X 10.6 or greater and is available for $34.95 from St. Clair Software at: www.stclairsoft.com/DefaultFolderX/

AppDelete
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AppDelete is for people who want to confidently try new apps while knowing they can uninstall them easily. AppDelete will remove not only Applications, but also Widgets, Preference Panes, Plugins, and Screensavers along with their associated files.
AppZapper requires OS 10.7 or later and is available for $7.99 from: http://www.reggieashworth.com/appdelete
You may also want to try the free programs AppCleaner at: www.freemacsoft.net/AppCleaner/

USBOverdrive
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USB Overdrive is a device driver for Mac OS X that handles any USB mouse / trackball / joystick / gamepad and any Bluetooth mouse from any manufacturer and lets you configure them either globally or on a per-application basis. It reads all kinds of wheels, buttons, switches and controls and assigns them to useful actions like clicking, typing, scrolling, launching, opening documents and so forth. USB Overdrive can easily handle several USB devices at once.
USBOverdrive requires OS X 10.6 or later and is available for $20 at: www.usboverdrive.com

TextExpander
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TextExpander saves you countless keystrokes with customized abbreviations for your frequently-used text strings and images. Use different email signatures?– Easily insert a customized signature with a few keystrokes. You can even include a photo or logo! Need easy access to boilerplate paragraphs?– Add them to your snippet library and insert them into your emails and contracts by typing a short abbreviation! Filling out forms?– Make it easy on yourself by adding your address, email, phone and fax numbers to your snippet library! Keep making the same typos?– Add common typos as triggers – TextExpander automatically replaces them with the correct spelling!
TextExpander requires OS 10.4 or higher and is available for $34.95 from: www.smileonmymac.com/textexpander/

DEVONnote
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Your thoughts are important! Treat them like gold with DEVONnote. DEVONnote not only keeps all of your notes safe in one place, but also helps you organize your ideas in a way that’s meaningful to you, using built-in artificial intelligence. DEVONnote treats websites like 'living' notes, allowing you to both view your favorite sites from within the application and manage your bookmarks like simple documents. It's finally possible to keep notes and related bookmarks together in one place!
DEVONnote requires 10.4 or later and is available for $14.95 (w/edu discount) from Devon Technologies: http://www.devontechnologies.com/

Caffeine
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Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers. Click it again to go back.
Caffeine is available for free at: www.lightheadsw.com/caffeine/

Easy Find
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Think Mac OS X's Spotlight could use some help? Download EasyFind, an alternative to (or supplement of) Spotlight and find files, folders, or contents in any file without indexing. EasyFind is especially useful for those tired of slow or impossible indexing, outdated or corrupted indices, or those just looking for features missing in the Finder or Spotlight.
Easy Find is available for free from Devon Technologies at: www.devontechnologies.com

TinkerTool
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TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows you to activate hidden features in the operating system and in some of the applications delivered with the system. The tool makes sure that preference changes can only affect the current user; you don't need administrative privileges to use the tool. The program will never change any component of the operating system, so the integrity of your system is not put at risk, and there will be no negative effect on system updates.
TinkerTool is available for free at: http://www.bresink.com/osx/TinkerTool.html
You may want to try Deeper, also available for free here: www.titanium.free.fr

Desktop Curtain
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There are times when you need to make a screenshot with no desktop clutter and/or with the standard desktop picture. Desktop Curtain displays your favorite desktop picture just in front of your real desktop, so you won't have to clean up each time you need to take a screenshot. And it's really easy to control — just control- or right-click on Desktop Curtain's desktop picture to access its settings or quit.
Desktop Curtain is can be used for free or you can unlock additional features for $5: www.manytricks.com

How to Find and Save $$$ on Shareware
MacWorld.com publishes a regular weblog titled MacGems where you can learn about some of the best low-cost mac software and hardware. Also, the following sites provide direct links to all the latest freeware, shareware and commercial Mac software: www.macupdate.com and www.versiontracker.com. Macupdate and the website MacZOT promote one program daily at a significant discount, but for 24 hours only.
Remember, shareware implies "try-before-you-buy", so if it interests you, test it out first, then decide if you think it's worth it. Other software is considered "donation-ware", so you really don't have to pay at all. In the case of the software "TextExpander", it was originally released as donationware and I decided to send the developer a few dollars via PayPal. When the product became shareware, I was sent a full license for free, simply because I had been charitable... :-)