OK, so the Android Market is more akin to Lidl or Asda than the iPhone's enormous, indulgent Selfridges-at-Christmas time approach, but the open source nature of Google's OS means there are plenty of apps for Android to be found.

1. Seesmic
There are many Twitter apps on Android - and Twitter itself shook up the scene with the launch of its own-brand app recently - but we're sticking with Seesmic. Offering support for multiple accounts, a home page widget showing latest tweets and an incredibly slick and professional design, it's one of the finest examples of app development out there today.
Seesmic
2. Facebook for Android
Facebook for Android is lacking in features compared to Facebook itself, but a recent update added Inbox support to the Android app, finally allowing its users to communicate in almost real-time. The app's fast and stable, with a simplicity that reminds you of the old days when using Facebook used to be bearable.
Facebook for android
3. ASTRO File Manager
ASTRO is nothing more than a Windows-style file explorer, but if you're into tinkering and directly installing Android APK files yourself, it's essential to stick something like this on your phone. Makes your phone feel like a computer, and makes you feel like you're in charge of it.
UPDATE: ASTRO still exists as a free ad-supported app, but you'll have to pay for the full version without ads. Our new favourite file explorer app is listed below.
Astro file manager