The built-in flash on Canon's Prosumer DSLR lines is generally considered
inadequate for most serious purposes.
The Canon accessory flashes such as the Canon Speedlite 550EX Flash
are much more versatile and powerful. The Canon Speedlite 550EX Flash gets
the light far above the lens (prevents half-moon shadows in your pictures)
and does a much better job preventing the red-eye effect.
The Canon Speedlite 550EX Flash allows bouncing the flash off of a standard height ceiling or off of other
reflectors such as white walls, white foamcore,
Photoflex and other commercial
reflectors, ... The Canon Speedlite 550EX Flash also allows the use of light
softening and modifying accessories such as
the Lumiquest Big Bounce,
the Lumiquest Softbox,
and the Sto-Fen Omnibounce.
Another benefit of Canon's accessory flashes is that they significantly help
focusing on many EOS cameras.
A pattern of the focus assist light projected by the flash helps the camera focus
faster and more accurately - even on a low contrast surface such as a white wall.
The focus assist feature can make or break a shot in low light situations!
The Canon Speedlite 550EX Flash is the largest and second most powerful of the Canon general purpose flashes.

Pictured above are
the Canon Speedlite 420EX E-TTL Flash (smallest),
the Canon Speedlite 580EX E-TTL II Flash (middle)
and the Canon Speedlite 550EX E-TTL Flash (largest).
The Canon 550EX Flash is substantially larger than the
Canon 420EX E-TTL Speedlite Flash,
has more power, and many more features such as manual power selection, FEC (Flash Exposure Control) and manual zoom settings.
Of course, it costs nearly twice as much as its sibling.
It is slightly larger, slightly less-powerful and somewhat less expensive than the
Canon Speedlite 580EX E-TTL II Flash.
Canon EOS 300D Digital Rebel
owners should take note to the FEC (Flash Exposure Compensation) feature
of the Canon Speedlite 550EX Flash.
The Canon EOS 300D Digital Rebel
does not have FEC built into the camera like most of Canon's other Digital SLRs.
The Canon Speedlite 550EX Flash will add a nice feature to your setup.
One feature of the Canon Speedlite 550EX Flash is that it can be a wireless Master
to trigger remote flashes (via light) such as the
Canon 420EX flash
and the 550EX.
The Canon Speedlite 550EX Flash can also be used as a wireless
slave to other Canon Speedlite 550EX Flash or to a Canon ST-E2.
I want to stress the importance of learning how to use Canon's E-TTL Flash system.
The PhotoNotes link in my "More Canon Speedlite 550EX Flash Reviews" section below will get you well on your way
to great flash photography results.
Canon Speedlite 550EX Flash Sample Pictures