How can I verify a signed message? I can send a signed message using the standard client, but if I see a signed message in, say, a forum, how can I verify it?

I understand this hasn't yet been implemented in the GUI for the standard client.
According to this bitcointalk thread, it can be done via the JSON-RPC API:
Code:

verifymessage [address] [signature] [message]


This feature has also been included in the GUI of the Armory client:


  To Verify a Signature Block with Armory (works fine in offline mode):

Open Armory, go to the menu, "Tools"--&gt;"Message Signing"
Click on "Import Signature Block" at the bottom of the screen.
Paste the signature block, and click "Okay"
Address will be checked against public key, public key checked against signature for the given message

  
  None of this requires the blockchain, so if you are on a system that
  couldn't normally run Armory, you only need to run with the
  "--noblockchain" option.  This allows you to verify Armory signature
  blocks without even having an Armory wallet!  If you're intrigued, try
  this one:  (get Armory if necessary)