These are recommended plugins. If you find a better one let me know.
Drawing - Gliffy
Prototyping - Balsamiq
jEvaluating
Spreadsheet Functionality in Tables
Comparison chart with key features:
| Using | Plugin | Math Formulas | Inline Editing | Works in Mobile | Auto-numbering | Comment |
|---|---|---|---|---|---|---|
| Yes | Bob Swift Advanced Tables |
|  | | | Like the most but requires math functionality. |
| Yes | Excellentable | | |  | (as far as I can tell) | Nice search function but ugly. |
| No | Spreadsheets for Confluence | | | | | n/a |
Encryption
Looks promising, no public key, but simple enough - https://marketplace.atlassian.com/plugins/com.prcpo.confluence.plugins.secure-content-block
Form Capture
http://confluence.atlassian.com/display/CONFHOST/Capturing+data+with+forms - seems to have a good run down of the available options.
http://blogs.atlassian.com/2009/05/confluence_wufo/ - expensive but looks to be very powerful and mature. Video demo on home page. Requires an additional server.
Workflow
Still looking, have not tried anything yet.