Push.ly is a site that pushes mentions of your username(s) and tweets of people you follow right to your mobile phone using the Notifo app, for free! Push.ly is also able to integrate with your favorite mobile Twitter clients so that once you receive a mention or tweet, you can open it right in your normal mobile Twitter app.
I actually forgot that Twitter would do updates on some users via SMS, because Push.ly is so awesome. Faster than SMS, and the developer responds quickly.
EDIT: Seriously. Push.ly + notifo is essential for anyone with an iPhone.
[Initial Review] Too bad that of my 13 users I track with push.ly, the only Twitters push.ly seems to push to Notifo appears to be the first 3 or 4 users on the list.
[Followup] It turns out that I was adding family members who had "protected" Twitter accounts, and push.ly can't follow those accounts, which makes some sense. I upped my rating accordingly since Push.ly does work on unprotected twitter accounts.
Functionally excellent, almost always extremely responsive and never given me any issues. Only falls short of five stars with me, due to it's rather unpolished ugly website. I love the product though!
Push.ly has very low latency updates and works every time. Great!