Great home repair website, and he has done reviews of gutter guards. Most are not effective (but are expensive!).
I personally have tried the cheap plastic ones, with mesh and without, from the big box stores, and hated them (didn't stay in, looked cheap).
My personal favorite is Gutter Brush. It is what it sounds like - like a giant bottle brush, that sits permanently in the gutter and fills the space with the radial bristles. I used this system for years and never had leaves, or maple seeds, get in my gutters. Not too expensive, either. Only problem is they may be a little visible (black bristles sticking up above the gutter a half-inch). Not a big deal to me, but my wife didn't like the look so we pulled 'em.