[Spider Girl]

Just as important (more) is what you're trying to accomplish with the mulch? Grass clippings (provided that they haven't been indundated with pesticides) about 4 inches thick are very effective at keeping weeds down, cheap, and compatible with all plantings. Salt marsh hay is good because it lets water through easily, but it can be hard to come by in some areas. Bark mulch is attractive, but costly (depending on the size of your garden) as well as acidic and tough for some of your more sensitive plants. A good way to get more out of bark mulch is to layer plain old newspaper underneath (it's very effective at keeping weeds down and disintegrates nicely).

Bottom line -- you need to understand the plants in your garden and what you are trying to accomplish with the mulch.