I whipped this up in about 2 minutes. If you want an extension where you highlight text and want to search StackOverflow with it, here is the Config.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Version</key>
<integer>1</integer>
<key>Extension Name</key>
<string>StackOverflow Search</string>
<key>Extension Identifier</key>
<string>net.ericd.stackoverflow</string>
<key>Extension Description</key>
<string>Open StackOverflow and search for the selected text.</string>
<key>Actions</key>
<array>
<dict>
<key>URL</key>
<string>http://stackoverflow.com/search?q={popclip text}</string>
<key>Image File</key>
<string>stacksearch.png</string>
<key>Title</key>
<string>Google Translate</string>
<key>Title</key>
<string>StackOverflow</string>
</dict>
</array>
</dict>
</plist>
Add a 256×256 .png with your icon to the folder. Rename it with a .popclipext & tada! You have your extension.