Tkinter

Pros

• Ships with Python.
• Also has C and Tcl bindings.
  

Cons

• Not necessarily packaged with Python.
• Looks like arse.
• Nobody has tried writing doco for it in 5 years.
• Nobody has ever written complete documentation for it.
• It's behind the mainstream GUI toolkits by about 15 years.
• The bindings were written in around 1997, and probably only stick around because they're low maintenance, and picking another toolkit would invoke a holy war.
• Errors, and invalid geometry configurations don't always throw exceptions, and may cause infinite loops.