I agree with the others that a low smooth tone has little to do with effects, unless by "effects" you just mean "pedals," in which case a good EQ would help (unless of course your amp already has a good EQ on it, in which case that would probably be redundant). A compressor, used SPARINGLY, can help to fatten up your tone a little. IME, modulation effects such as chorus actually tend to brighten up my tone rather than making it sound low and smooth, but then again, that might just be the particular pedals I've used.
Strings can also make a world of difference, probably more than effects. A few people mentioned flats, and that's good advice, depending on what brand you get (as I understand it, even some flats can still be pretty bright). Half-round (aka "ground wound") strings might serve as a happy medium if you're not interested in flats for any reason.
And of course, technique - always technique. Playing closer to the neck will generally sound smoother than playing near the bridge. Although some think it heresy, playing with your thumb might work too - as I understand it, a lot of the early electric bass players did just that, in order to get a tone closer to a smooth DB jazz tone.
Hope that helps.
