Need to plot some data quickly? Try plotmydata.blogspot.com.
Your data is plotted in real time as you type, or you can copy/paste in your data in csv format.
This is for the times when you don't want to wait for Matlab to open, and it's not worth it to write a python matplotlib script/use a command line tool.
What features do you want - uploading text files? scatter plots? zooming? comparing multiple trends? plotting an arbitrary function? anything else?
I'm all ears.
Just one of those projects. It makes use of Stanford's protovis library.