I had previously mentioned JDocs on my blog. JSourcery provides a similar service. The site currently comprises the Javadocs for several common APIs (such as the Apache Commons APIs), as well as the actual J2SE Javadocs (although I suspect it is only a matter of time before SUN will force JSourcery to take these down, just like JDocs did). The unique thing about JSourcery is that it also provides access to the hyperlinked, highlighted source code. On the other hand, it does not have JDoc’s comments feature, and the list of APIs is significantly smaller.
In my opinion, these two services should merge. JDocs has probably gained a fair amount of traction, and the added source code feature would be a nice addition.