Darren Green of SQLIS.com has posted a new SSIS Expression Tester on Codeplex: http://expressioneditor.codeplex.com/
See the SQLIS blog post from yesterday (6/9/10): http://www.sqlis.com/sqlis/post/ssis-expression-editor-tool.aspx
German-language blog post from Tillman Eitelberg at ssis-components.net: http://ssis-components.net/post/2010/06/09/Expression-Tester-Tool.aspx
Crank this up along with your expressions Cheat Sheet from PragmaticWorks and the tools for expressions in BIDS Helper, and reduce the number of 4-letter "expressions" that you have to use when working with this valuable SSIS feature.