This short Applescript prompts you for a Wikipedia topic and opens the resulting page in Safari. There is a nice Wikipedia Dashboard widget, but sometimes you might want full browser access. You can save the script in the Script Menu (~/Library/Scripts/), so it's accessible no matter what application you're in. Wikipedia is smart enough to convert spaces to underscores in the URL, so it's no problem if your topic has spaces in it.
-- Wiki-Lookup
-- Quickly look up an item in Wikipedia
on run
set Entry to display dialog "Lookup a Wikipedia entry:" default answer ""
set textEntry to text returned of Entry
set theURL to "http://en.wikipedia.org/wiki/" & textEntry
tell application "Safari"
activate
make new document with properties {URL:theURL}
end tell
end run
kirkmc adds: If you don't see the AppleScript menu in your menu bar, open AppleScript Utility (located in /Applications/AppleScript), and check Show Script Menu in menu bar. If you use a different browser, you can change the "Safari" in the script to the name of your browser.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070528090709539