The Q-bond above sounds good, I've used similar product with good results Henkel Stabilit Express and there's another: Uhu Acrylit. Fills gaps too and you can sand it to a smooth finish.
But it really depends on the application. Plain 2-component Epoxy adhesive (Araldite) works too, best applied in a thin layer between good fitting components. I also like Automotive PU elastomer adhesives (think of the stuff used to glue windshields in cars) applied in thicker strands and allow for some give/flex after curing. They're single component usually and packaged in a tube like bathroom kit.
surface prep is the key to success for all of them, but the PU adhesive is most forgiving in my experience.