iPhone only: As its name suggests, free iPhone application Voice Dialer adds voice dialing to your iPhone. More accurately, though, Voice Dialer is a contact search-by-voice app that also does autodialing. The difference: You can easily autodial any contact by saying "Call John Smith at home"—and Voice Dialer is great at recognising matches—but if you don't say "Call" before the contact's name, Voice Dialer will simply pull up matches. From there, you can also check out a contact's full contact card to quickly compose an email, send a text message, or launch Google Maps at your contact's address (the video demo illustrates these features nicely).
Voice Dialer is made by the same people who developed midomi, an excellent song-recognition engine that can identify a song by holding it up to the source while it's playing or by analysing your attempt at humming a few bars; you'd imagine the technology behind Voice Dialer and midomi are fairly similar. Voice Dialer is a free download from the iTunes App Store. For a similar alternative, check out previously mentioned Cactus, a less polished voice dialing app.