Some noteable features:
- A unique user interface.
- Works on VGA, QVGA and SmartPhone devices.
- Support for multiple twitter and identi.ca accounts.
- User groups to help manage your friends updates.
- "Conversation View" shows a what this status was in reply to.
- Uses standard notifications to alert you of new frields statuses or replies.
- Keeps an addressbook to easily add usernames to posts.
- URL shortening with is.gd.
- Integration with shorttext.com for tweets longer than 140 characters.
- Take photos and send to TwitPic.
- Preview images from TwitPic, mobypicture or yfrog without a browser.
- Integrated search.twitter.com.
- Quote (re-tweet) or email a status.
- Clickable @names, URLs, and #Hashtags .
- Option to check for new version on startup.
- Themes.

Requirements:
- Windows Mobile 6 (it may work on 5, I'm not sure).
- .NET Compact Framework 2.0 or better.
- A network connection.
- A twitter or identi.ca account.

If you like this tool, please consider donate to the developer.

Update Description:
v0.75b:
- CAB Installer for your phone.

v0.74:
- Fix existing image upload.
- Fix several crash reports.

v0.73:
- Fix issues with media services and other bugs.

v0.72:
- Fix @mentions and DMs.
- Other bugfixes.

v0.71a:
- Grouping, Today Screen Plugin, New Picture Services.

v0.70:
- Fixes blinking avatars and other bugs.

v0.69:
- Fix smartphone pictures, DM Avatars, and crash with old themes.

v0.68:
- More timelines, Multiple Picture Services, Addressbook.

v0.67:
- "Unread" counts.
- Minor bugfixes.

v0.66a:
- Lots of bugfixes.
- Cache folder setting.

v0.65:
- Bugfixes.
- Minor menu improvements.

v0.64:
- Adjustable font size.
- Fixed "stuck" pop-ups.

v0.63:
- QuickPost.
- GPS for all locales.
- Improved battery.

v0.62:
- Fix my blank timelines from v61.

v0.61:
- Smartphone Notifications.
- Communication Error notifications.
- Bugfixes.

v0.60:
- Fix audio notifications in WM6.
- Fix Search and Post screens on VGA.

v0.59:
- Bugfixes.

v0.58:
- More maps.
- Improved Search.

v0.57:
- Fixed auto-update.
- Advanced settings view.

v0.56:
- Fix credential verification.
- New profile page view (with Google Maps).

v0.55:
- Improved stability.

v0.54:
- Fix GPS freeze, Low graphics memory error, and more.

v0.53:
- Better VGA support.
- Improved detailed view.

v0.52:
- Detailed view.
- Easier Ping.fm account setup.

v0.51:
- More bugfixes and optimizations.

v0.50:
- Fixes.

v0.49:
- Faster, Prettier.

v0.48:
- Fix a few bugs.

v0.47:
- Bugfix for reply ids.

v0.46:
- Conversation view, Hashtags, Themes, and more!

v0.45:
- Skweezer option, Quote function.

v0.44:
- Shorttext.com integration, better caching, smartphone account management fix.

v0.43:
- Copy+Paste in status update.
- More crash fixes.

v0.42:
- Less invasive error reporting.
- Don't get timelines from Ping.FM.

v0.41:
- Ping.FM fix.
- Selection fix.

v0.40:
- Avatar updates.
- No longer jumps to top on new tweets.

v0.39:
- Direct messages and replies shown under "Messages" menu item.

v0.38:
- Fix rendering issue for some clickables.

SOURCE

http://code.google.com/p/pocketwit/

Reviews:

***** [Apr 25, 2009] by Ayman Khateeb using HTC Touch Viva
This is by far the best Twitter Client for WM. It even beats many Desktop versions. Very very nice interface, finger friendly, and has a complete set of features. I really like it and recommend it.