The Command-Control-D shortcut to look up the word that's under your mouse cursor is very convenient -- much more so than having to open the definition in another program. You don't even have to select the word first. It's not an OmniWeb-specific thing; it's a feature of Tiger, and it works in many different programs.
If you keep holding Command and Control after invoking it, you can also move your cursor around and the definition will appear under any word you point at. It's even smart enough to look up phrase definitions when it notices two words in a row that have a definition entry as a phrase (for example, point at the word "Command" above, and you'll get a definition for "Command and Control").
You can also switch between dictionary and thesaurus, and there's a button to take you to the full Dictionary program if you are so inclined.
In short, this feature rocks, and I'll take it any day over having to first select a word, right-click it, choose a command, wait for another program to launch, then view the definition in another window.