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JSoftware's wonderful J programming language flips back and forth between being free and having some kind of fee associated with it. The latest version 5.03 has reverted back to a price of $0, though you need to request a (free) license key via email. This is one of the great programming languages, being directly from the mind of Ken "APL" Iverson. (JSoftware is essentially Ken + Roger Hui + Ken's family). If you've never programmed in anything APL-like, prepare for a mind-warping experience. It runs under Windows, Linux, and OS X.
For anyone interested, here's how the "How much does J cost?" issue has gone since about 2000 or so, chronologically:
1. A one-time flat fee to use J: ~$250 for an individiual license, ~$600 for a commercial one. A number of features were suppressed until you bought a license.
2. Free, plus no more suppressed features. I think you could still buy a license if you wanted, but this wasn't required.
3. Annual license fee: $100 for individuals, ~$600 for commercial use. Free evaluation licenses.
4. Free license key, requested via email.
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J is free again (as in beer)