Usability 
WebMD.com is the leading US based health and health care information site based on Comscore's anaultics. The mobile version of WebMD features a free Symptom Checker iPhone app along with a drug database, interactive pill identifier and comprehensive First Aid reference, all of which are web based. Provided you have a suppored handset WebMD is an good resource for on the go medical information.
The right handset part is my main annoyance with this site. WebMD's mobile product manager as clearly been sipping the iPhone Kool-Aide a bit much. Not only is the site's marquee feature a single platform app but the web based features requires JavaScript. There are other cross platform issues too, the site's First Aid and pill identification pages come up blank in the WebKit based Nokia N95 browser. With Opera Mini and Mobile the site has some minor formatting issues but is at least usable. On the plus side everything, except the iPhone App, does work perfectly and look great in the Android 2.0 browser.
WebMB is the sort of mobile web app that really should work across virtually the entire range of mobile browsers. Javascript based AJAX is great for improving performance and usability on advanced platforms but it really doesn't take much extra work to provide html-only fallback for more basic browsers.
I'm a little disappointed with WebMD's pill lookup feature too. It lets you search for pills by size, shape and color but presents results as a just a list of drug names with no images. For a mobile pill identifier, that includes images and works on all devices, take a look at Drugs.mobi.
Source: Taptu Touch Web Report (PDF)
May-2013: No longer has a mobile wepapp