For the last event we went to, I did up a veggie tray with 3 dips, hummus, black bean, and cashew-nooch-cheez stuff that I called nippy cheez dip.  which was basically melty cheeze http://vegweb.com/index.php?topic=10618.0 , smoked paprika and hot sauce.

;)b  you could try that, or a green salad, potato salad, pasta salad, heck throw some whole cooked chickpeas into a pasta salad and watch it disappear.

that's for larger gatherings.  for smaller (6-8 people) I like to make stuffed mushroomshttp://vegweb.com/index.php?topic=4747.0, or stuffed peppers http://vegweb.com/index.php?topic=22241.0, or stuffed onions, which always seems to wow people. 

Last night I went to a book club meeting where I was the only veg.  I brought vegan potstickers but didn't tell anyone they were vegan.  They were the hit of the party.  I had people coming up to me all night saying" OMG those were so good, I think I ate 12 of them."  I made 50 potstickers and they were gone by the end of the night (15 people).

They do take about 1-2 hrs to make though.  If you want something a little easier, salads such as tossed salad, pasta salad, or potato salad is good and easy for groups.  Last Saturday I made a vegan BBQ "ranch" pasta salad, and at a 4th of July party I made a pesto potato salad, taking advantage of an abundance of basil this time of year.  A few weeks before that I took a Mexican tossed salad with an avocado based dressing to a friend's house.  All of these were liked by the omni crowd.

If you want to bring a hot dish, you could do a casserole such as broccoli and rice, or a vegan lasagna, or some sort of taco filling with tortillas/shells, such as sweet potato and black bean.