As befits a camera at this level the G10 has a wide range of accessories for the user to choose from. These include Canon Speedlights, a teleconverter, and a underwater housing (surprisingly uncommon on cameras of this type).
A dedicated flash such as a Canon Speedlight allows the user much more control over flash functionality, including bouncing off ceilings and walls, which produces a much more pleasing effect than direct flash. All current Speedlights also offer automatic zoom functionality with the G10, matching the zoom setting of the camera with zooming in the flash head. Also available (but not shown) is the Canon ST-E2, which allows users control over multiple sets of wireless off camera flashes (extremely useful for portrait lighting).
The G10's hot shoe is compatible with all Canon's Speedlight flash guns. Here it is pictured with the 430 EX II. While it is a nice feature to have, the larger EX flash guns are so much bigger than the G10 that it is almost impractical.
If you own an EOS DSLR with EX speedlights, the G10 will integrate quite well into your system.
The TC-DC58D multiplies the focal length of the G10 by 1.4x, which at the longest zoom setting gives you 196 mm (equiv.) - more or less matching the G9's native telephoto capability. The adaptor requires the conversion lens adapter LA-DC58K be attached to the camera first (sold separately from the teleconverter).
Underwater housings are quite a specialist tool, which is why many camera manufactures don't produce one for their compact cameras. There is one available for the G10 (WP-DC28), which is rated to 40 meters or 130 feet.
While not a cheap accessory at $240 (£180), the underwater housing is very well designed. All the functionality of the G10 is available while inside including all the dials and buttons, though it is a little difficult to find the half press point of the shutter button.
On dry land, the underwater housing can have the side benefit of turning the G10 into something of a Fisher-Price toy, sure to draw smiles from anyone you point the camera at.