Here is a comparison matrix to see which is best suited for you: using Sigasi as an Eclipse VHDL plugin or downloading it as a standalone VHDL editor.

Standalone Sigasi Pro Eclipse
Separate Eclipse download
Eclipse versions Bundled with Eclipse 3.7 Eclipse 3.6 or higher
Compatible with Eclipse plugin mechanism
Recommended without prior knowledge of Eclipse
Recommended if you also work with C, C++, Java, …
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Installation instructions Download it Plug it in