Agile Messenger

http://www.agilemobile.com/agile_messenger.html

Pros
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+ Best interface, easy to switch between IM windows and see when a new message has arrived
+ Shows when someone is typing
+ Has an integrated and easy way to take and send pictures/audio recordings through the IM (Really cool!)

Cons
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- No custom Away Messages
- Can't check other people's away messages


IM+

http://www.shapeservices.com/en/prod...php?product=im

Pros
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+ Custom Away Messages
+ Check your buddy's away messages

Cons
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- Clunky interface, viewing new IMs and managing multiple windows is a pain


AIM 2.0

http://forum.xda-developers.com/showthread.php?t=271468
Here is the hacked version thats compatible with WM5 (Forth post)

Pros
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+ FREE!
+ Check your buddy's away messages
+ Custom Away Messages

Cons
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- Can't type with your away message on
- No custom aliases (if the screen name is "SuperHotMamma7865", you can't change it so you just see "Mom")
- Signing in will automatically sign off all other instances. So if you leave your IM running on your desktop at home, it will be signed off
- Doesn't automatically reconnect after a phone call
- Managing multiple IMs is a pain
- No history (doesn't automatically save IM conversations)