What is a good dressing for bean salad?

All these suggestions are good.
If making in quantity, a simple Good Seasons Italian Dressing made with quality olive oil and (we like a) slightly higher % of vinegar. You can add extra spices to match the meal- cumin for Mexican, etc.

For baby lima beans, I use olive oil, lemon, oregano, salt pepper..Let that sit a few hours and then mix in some chopped parsley and feta cheese.

Warm bacon vinaigrette with parsley

Sam1148 said it. Its usually oil, and acid to taste, season with fresh herb (usually cilantro or flat leaf parlsey) and salt / pepper. If you want additional info, we need to know the beans. Some spices are used as well, the most common in my opinion is cumin (with chopped onion, garlic, cilantro, oil and lemon juice).

Simple olive oil and lemon, with some crushed garlic.
It really depends on the beans you're using...can you expand on that?