Best tool for quickly cutting vegetables into a salad?
You could chop all your veges with a knife once a week and put them in containers...

If you are set on a machine, pick up a Food Processor (like a Cuisinart).  It has attachments to slice or grate your veges quickly and uniformly, and has many other uses too (quick bread dough, sauces, puree soups, etc.)

A Mandolin slicer will also work, and you can julienne things, but make sure to get a good one.  I am not happy with mine at all.  The advantage to this is you can change the thickness of your slices (i.e. thick cucumber slices, thin radish slices).

The tomatoes you will just have to cut by hand though.  There are tools for that, but I haven't seen a consumer version (giant slicers/wedgers at McDs, many decades ago).