The Centro comes absolutely loaded with great 3rd party apps, right out of the box. This is great news for consumers, as it means some cost savings and less time doing setup. Here are my personal favorites included in the ROM:
Google Maps: Google maps is grand. It's great getting directions and traffic information and the interface is snappy and easy to use. It's not up to the iPhone's Google Maps app, but it's still very very good.
Documents to Go (Version 10.0): Documents To Go handles the office documents (Word, Excel, and Powerpoint, and straight text) for the Centro and it does a great job of it. You can view, edit, and create office documents and sync them relatively easily back to your desktop. You can also use it for email attachments. Most surprising - the Centro is the first smartphone to support Office 2007 documents - something even Microsoft's own smartphones cannot do yet. DataViz deserves a ton of credit for their wonderful series of "To Go" applications. They can't handle everything that an office document can throw at them, but they can handle tables, some charts, bullets, and more without scrambling up the desktop formatting.
Astraware Sudoku: I prefer crosswords myself, but I know I'm in the minority lately. Sudoku is built in (instead of Bejeweled :( ) and many people will be happy about that.
Pocket Tunes Deluxe (that's the full version): Palm has finally realized that they need to get really, really serious about media on the Centro and so they've begun including the full version of Pocket Tunes deluxe. This means internet radio streaming, which is incredibly cool and I'm happy to see it.
SprintTV: SprintTV is also included and a nice app, if a bit choppy with slow connections.
Nuance Voice Control: Nuance is an incredible app. You record a short command and the Centro does it - "Call Jim Mobile" calls Jim on his Mobile phone, "SMS Jim Sorry I can't talk right now, I am driving" automatically opens up a new text message and fills it in with that text. It's super cool, although it does require that you upload your command to Nuance's servers in order to work so there's a slight delay. It's also prohibitively expensive at $7 a month.
IM: The built-in instant messaging client works with AIM, Yahoo, and MSN. I have to say, though, that there is a significant lag time in sending and receiving messages. I haven't been able to determine yet whether or not this is due to my relatively poor coverage, but anecdotal reports from a few others imply that the delay is a real problem everywhere. On the bright side, the application is clean and fast and works in the background.
On Demand: Sprint's On Demand application (powered by Pocket Express) is also built-in and pretty nice - giving up to date information on news, weather, stocks, and more. I generally find it to be a little resource-heavy and don't particularly like the UI myself, so it's not something I'd pay to get the full version of. Some folks like it though, so more power to 'em.