Quick and Easy Shelf Techniques?

Today I am building shelves. Let the excitement begin!

We need two sets of shelves: one in the bathroom and one in the kitchen. I’m not doing anything ornate as we just rent. (Our landlord would probably shit a brick if I did anything more than screw them into the walls for support.) I don’t have a plan for materials, though, and am just planning on running over to Lowe’s and seeing what strikes me.

What has been your experience with shelving systems? I know there are piecemeal systems available at the big hardware stores, complete with finished lengths of particle board. The last time I looked at those they seemed to be really expensive. I’m also not sure the boards will be in the depths or lengths I need—I’m going for shallow and slender—and there’s nothing that looks junkier than cut particle board.

I’m not opposed to doing some actual cutting and painting, although I don’t have the equipment to route out slots for shelves to fit into, so I’m not sure about buying only lumber and trying to build it up from there. (I’ve got a drill and a circular saw and a saws-all; I don’t want to buy any more tools at the moment.)

Any suggestions?