Nana and I are halfway through the process of finding an apartment (aka, a flat) in Edinburgh for the year--we've done the web research, but won't be viewing properties until next week--so I thought I'd provide a kind of annotated bibliography of the sources we've used.
LettingWeb: Extensive coverage for Edinburgh and Glasgow; somewhat spottier coverage for the rest of the UK; extremely easy to use.
LettingWeb is to flats what Kayak is to airfares: LettingWeb compiles listings from a bunch of other sources. The site has an incredibly useful Map Search feature, which is great for newcomers who aren't familiar enough with the city to navigate the other listings easily--though Map Search does occasionally duplicate listings, and listing locations are sometimes off by as much as a block.
The disadvantage is that LettingWeb sometimes overlooks pictures posted on other listing sites--especially those operated by individual property management companies--and because it seems to scan only professional listings, it occasionally misses properties being let directly by the owner.
CityLets: Not as extensive as LettingWeb; sometimes has more pictures; more likely to include properties being let directly by the owner.
CityLets is harder to use--no map search--and less comprehensive than LettingWeb, but often has more information on properties listed by LettingWeb. Occasionally, a private-let apartment will show up on CityLets and not on LettingWeb.
Gumtree: Difficult to search; almost exclusively private-let; most listings appear neither on LettingWeb or CityLets.
Gumtree is the UK's preferred alternative to Craigslist. We spent a couple hours scouring the listings here only to find three or four flats we liked, all but one of which had already been let. (Users don't seem to take their posts down after they're obsolete.)
Gumtree, being free, is also a favorite with scammers. Basically, if it looks too good to be true, it is--and even then, you're not safe. I replied to an ad for a flat listed at a cheap, but not ludicrously cheap, monthly rent and was rewarded with a gloriously scammy e-mail from "Mrs. and Rev. Brad Pitt."
For tips on avoiding rental scams, check out this FTC website.