I made a couple of minor edits to my Wiki page about online basic calculators, scientific calculators, graphing calculators, CAS systems, and plotting utilities today.
It still blows me away how much is available online and how good the quality is. Yacas (yet another CAS) has a tab called My Yacas that provides quick access to the list of commands and to the actual CAS engine - no download required.
Now, if only math students had the kind of access to computers and the internet that would allow them to use these tools routinely in school.
Update: Don't miss the link above to the actual page listing online calculators, CASs, graphing utilities etc.