Pud has a good post on brands and sub-brands, pointing out that Flickr and YouTube are much stronger, brand-wise, than Yahoo! Photos and Google Video, which may relate, at least a little bit, to their success. This is an excellent point and one of the things startups have as an edge on Google and Yahoo.
Not to be nit-picky, but I would define the terms a little different than Philip: Gmail is a sub-brand, where as Yahoo! Mail is just using the parent brand. I don't know that this is a technical definition (and there are professionals in this realm who would tell us if we ask, and probably will even if we don't). The difference is whether or not there's a "brandable" name tied to the product, but overall still a close association with the parent brand, versus no attempt to brand the product (i.e., using a generic descriptive name) apart from the parent brand. So, you really have three choices when you start a new product: Use the parent brand (Google Reader), use a sub-brand (Gmail), or use a stand-alone brand (Blogger).
It's clear that Google has no real strategy along these lines. The default seems to be that they use the parent brand (Google Video, Images, News, Maps...) unless they can think of a clever sub-brand name (Gmail, Froogle), or unless they acquire it, in which case they tend to leave it alone (except in the case of Keyhole, now Google Earth). AdWords and AdSense are two fine sub-brands (and two of their most popular products), which don't fit this pattern. Then there's the case of a homegrown thing they didn't know if they want to be associated with, so they stand-alone branded it.
Yahoo! used to have a pretty clear strategy: Everything used the parent brand (Personals, Finance, Travel, Photos, Podcasts, Search...zzz), until recently when they've realized that maybe some acquisitions are better left with their own brand and maybe even some new homegrown things should be a little less generic. And in at least one occasion they co-branded an acquisition, so you'll notice its Yahoo! Hotjobs, not just Yahoo! Jobs (which is probably correlated with the purchase price).
Apple, of course, is the master-brander with a super-strong parent brand. And, you'll notice, they sub-brand almost everything: iPod, iTunes, iWork, iLife (a sub-brand family, if you will), PowerBook, Mac, .Mac, OS X, G5, MacBook, AirPort, etc. Everything is distinguished in its category, as well as unmistakably linked to Apple. (Imagine if they called it the Apple MP3 Player?)