Most DSLR cameras have small flashes integrated within their bodies, and so you may not feel you need one if you're just starting out. Nevertheless, there are many reasons why you may at some point go for a separate flashgun unit.
These are far more powerful than the ones included in DSLRs, and those with tilt and swivel functionality are more flexible in relation to how their light is output. They can also be used off-camera for a more flattering illumination, as well as in combination with other flashguns for multiple-flash setups.
The cheapest flashguns may be limited in terms of how their heads may be positioned, although they will still provide more power than a built-in alternative.
More expensive models come equipped with all sorts of niceties, from backlit LCD screens (useful in low light), to more expansive focal range coverage than cheaper models, as well as more precise control over their output.
These will also typically recycle faster and with less noise, and some may even have an integrated USB port for future firmware updates.
Of course, all of this comes at a cost, and some command asking prices similar to that of a new lens.
Fortunately, there are a range of third-party flashguns from the likes of Metz, Sigma and Nissin, which compare favourably with more mainstream alternatives in terms of both specification and performance.
If you do decide to go for a third-party option, however, make sure that it supports your cameras TTL system. You should be able to find this information on the flash manufacturer's website.
Jessops 360 AFD - £79 (about $124)
A dedicated alternative for Canon, Nikon and Sony DSLRs, this flashgun has a fairly limited feature set, although the basics are pretty well covered. The motorised zoom is quite slow, as is recycling speed, but the flashgun works effectively as a light-sensitive slave unit.
Polaroid PL-144AZ Power Zoom Flashgun - £99/$120
Polaroid isn't known for its flashguns, but this sub-£100 model does much to impress. It offers a capable guide range of 45m at ISO100, and a head that zooms between 24-85mm, together with 90-degree tilt and 270-degree swivel functionality. Unusually at this price, it also finds space for a backlit LCD display. In use, it recharges quickly and responds promptly to changes in camera settings, and its illumination is good. Sadly, it's fairly noisy both when recharging and zooming.
Sunpak PZ42X - £99/$145
Built in Canon, Nikon and Sony options, this dedicated flashgun builds motorised zoom, bounce and swivel, autofocus assist and TTL metering into a neat and compact package. There aren't any wireless master/slave functions, but the Sunpak is a solid performer that's very easy to use. It's a bargain at the price.
Nissin Speedlite Di866 Pro - £181/$289
Made in Canon or Nikon dedicated options, this is one of the best flashguns on the market, with a nice LCD display, plenty of power and a huge range of upmarket features, including full master/slave wireless operation. A secondary sub-flash is good for fill-in lighting and there's an optional quick-change battery cradle.
Canon Speedlite 430EX II - £188/$279
This Speedlite is easy to use, with direct access to advanced modes such as high-speed sync and rear-curtain flash, along with super-fast, silent recycling circuitry. It also works well as a wireless slave in multi-flashgun setups, but maximum flash power is a little on the low side.