ScreenRecycler lets you use an old iMac or laptop as a secondary display.
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Macs are great computers. One of the best things about them is the wealth of inexpensive applications available to help you get more from your investment.

Third-party Mac apps can help you keep track of your to-do lists, manage text and passwords, run Windows on the Mac, recycle old computers for use as displays on new computers, and more.

And you don't have to pay an arm and a leg to get these applications -- many of them are surprisingly cheap.

We've put together this list of 19 of our favorite cheap and free Mac apps, to help you get more done with your machine.

1. ScreenRecycler
Imprint / Impressum
$29.90

A few weeks ago, I bought a new 24-inch iMac, leaving me with my old machine as a spare. The old machine is a 2½-year-old iMac, also with a 24" display. Ultimately, the old iMac is going to become a media-center PC, but until then I'm using it as a secondary monitor for my main desktop, using ScreenRecycler.

Many of us have obsolete laptops, iMacs, and other all-in-one computers lying around. They still run, but they're too underpowered to use with today's software. ScreenRecycler is one way to put those obsolete computers to work.

ScreenRecycler lets you run another computer as an additional screen on your primary computer -- it's like having an additional monitor attached to your computer. Your primary computer, the one that runs ScreenRecycler, needs to be a Mac, but the second computer can be a Mac, Windows, or Linux system. Both computers need to be on the same local network.

I've been using ScreenRecycler for weeks now, and it works great. Occasionally, response time lags by a second or two, and I wouldn't want to run a graphics-intensive game on the secondary monitor, but mostly ScreenRecycler just sits there showing me instant messages, Twitter, iTunes, and other apps that I want to keep in the corner of my eye.

2. Things
Cultured Code
$49.95 desktop / $9.95 iPhone

Things is an attractive and easy-to-use way to manage your to-do lists. It's modeled loosely on the Getting Things Done philosophy of David Allen, but you don't have to be a disciple of The David to benefit from Things.

Things lets you enter to-dos and check them off when you're done. The software organizes your tasks using three major lists: "Today," for things you want to get done that day; "Next," for things you want to get to soon; and "Someday," for things you'd like to get done at some point in the vague future.

You can also organize your to-dos into projects and areas of responsibility. Things lets you assign due dates. You can set tasks to recur by day, week, month, or arbitrary time periods like every other week.

Things also allows you to tag your to-dos, and then sort the tasks by tag. I like to tag some to-dos "first thing." When I get to my desk in the morning, I check out the day's tasks that are tagged "first thing," and I get right to work on them, knocking them off before I do anything else.