SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering.

WiFi Scanner is the most feature rich and complete WiFi scanning app on the app store and is a great tool for personal use and for IT professionals.

FiveNotes (formerly known as Quick) is a small text editor that you can always access by clicking (or pressing shortcut keys) on its mtatus-bar icon, even if you're working in a full-screen app. If you prefer, it can stay on top of the window so it can be used as a cheat-sheet app. FiveNotes offers exactly 5 notes that can be switched using colourful circles under the text area or by pressing shortcut (Cmd+Shift+[ and Cmd+Shift+] or Cmd+number-of-note).