Well, not entirely limited to libraries.  Useful stuff for visualization practitioners sounded a little non-specific, though.  These are all freely available.

1. Prefuse (Java) & FLARE (Flex) 
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 2. simile (AJAX)

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 3. Processing (Java)

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4. GigaPan (Service)

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5. Modest Maps (Flash, Python)

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6. Google Visualization API (Javascript)

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7. Google Chart API (Javascript)

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8. Google Maps API (Javascript, Flash)

9. GraphViz (Wrappers for a dozen languages including Java, Perl, Python.  Free.) 

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10. JFree (Java)

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11. pChart (PHP)

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12. OpenLayers (JavaScript)

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13. Anti-Grain (C++)

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14. JGraph (Java)

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15. Boost Graph Library (C++, phyton wrapper)

16. Open Flash Chart (Flash)

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17. Ubigraph (Wrappers for Python, Java, C, and more)

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18. JUNG (Java)

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19. TimeMap (Java)

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20. Many Eyes (online service)

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