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***Completely Free***

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Papel (Windows) is abandonware. The link is to its new home at a mirror site.
Features here, 1 page (of 7) of screenshots here, download here.
Bookmark Romanzo for future use. It’s in the planning stage, but once finished, it claims “it will offer much of what Papel promises but with several key advances.”

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Text Block Writer (Windows) is abandonware. (The link is to Softonic because the developer’s website is no longer running.)
Screenshots here.

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yWriter (Windows, Linux)
Screenshots here, Facebook page here.
Check out this (awesome) guy’s other software while you’re at the yWriter page. (Awesome guy being Simon Haynes.)

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***Less Functional Free Version of Otherwise Commercial Software***

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Celtx (Windows, Linux, Mac) – Doesn’t include add-ons.
Overview here, features here, screenshots here.
Full version $14.99.

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Page Four (Windows, unofficially/maybe Linux) – Restricted to 3 or 4 notebooks rather than unlimited.
Demo video and screenshots here, FAQ here, download here.
Full version $39.95.

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Spaaze is an ‘infinite virtual corkboard.’ I haven’t tried it out, but I’ve heard good things from fellow writers about it as a project management program.
Full version $3/month.

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Storybook (Windows, Linux. Mac option on FAQ page.) – Comparison of commercial and free version here.
Screenshots here, download here. Full version $25.94.

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SuperNoteCard (Windows, Mac, Linux) – Limit to the number of cards you can use, but I’m not sure what the limit is. I’ve used this program quite a bit before and didn’t run out of note cards, though.
FAQ here.
Full version $29.

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Writer’s Cafe (Windows, Mac, Linux, FreeBSD, Solaris) – Free version restricted to 20 cards and 20 scraps and doesn’t include e-book.
Features here, FAQ here, screenshots here, download here.
Full version $40 (or $60 for CD-ROM version).

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***Fully Functional Time-Sensitive Trial Version***

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Liquid Story Binder XE (Windows) – 30 Day Free Trial (30 days of actual use.)
Screenshots here, tutorials here.
Full version $45.95.

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Scrivener (Mac) – 30 Day Free Trial (30 days of actual use.)
Features here.
Full version $45.
(Free Beta version for Windows here. Will be $40 when it goes on sale.)

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Storymill (Mac) – 15 or 30 Day Free Trial (Scroll down to Storymill icon.)
Screenshots, video tutorials, and user guide on home page.
Full version $49.95, or $24.95 to upgrade to Storymill 4.

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Writer’s Blocks (Windows) – 21 Day Free Trial
Quick look here, features here, FAQ here.
Full version $149, or $99 to upgrade from Writer’s Blocks 3 to WB4.