I've been using Alfred.app for a few years now and find it extremely useful. It's most useful for:

• Quickly accessing folders or files nested deeply on our file server
• Pulling up frequently used webpages, launching searches on webapps and other web tasks
• Quickly launching a specific build or beta version of our apps

Alfred v2 has just been released and it improves nearly every aspect of the app – workflows are a particularly great improvement over the plug-ins in v1 (grab my updated "Open app in language" workflow on the projects page).

Check out Alfred.app – there's also a free version to try out first.