Quick Adium tip:
When writing a message, you can use the keyboard shortcut ⌃S (for swap or switch, I reckon) to switch between two buffers.
This is very useful if you are in the middle of composing a long message and need to reply to something else right away. Hit ⌃S and the long message is swapped out. Write your reply. Send it. Hit ⌃S again and the long message is swapped back in.
A down-pointing triangle in a circle in the text area seems to indicate that the other buffer has content.
If you need more than two buffers, you can use ⌃↓ (push this message onto a stack) and ⌃↑ (pop the top message from the stack), though this is probably rarely useful, and quite confusing.