How secure are your GUID / UUID (Universally Unique Identifier)?

@txyoji made an interesting observation in one post regarding security of GUID / UUID - you've probably seen these kind of strings in use when resetting a password or getting a 1-time token for something de305d54-75b4-431b-adb2-eb6b9e546014

This topic is for discussing GUIDs in different languages to find out which ones are secure and which ones aren't.

See this Wikipedia for more information.