I had real good luck this past year with portulaca (moss rose) planted around my mailbox garden. They didn't need any water and bloomed profusely all summer.

I have Clematis around mine. It needs a little bit of extra support, some of those wire supports with the round part at the top. I have three, all different sizes like steps. Stacy

bronze leaf begonias. ~Ryan

Also think about daylilies interplanted with irises!

We are going to tackle the mailbox this summer as well.I am considering placing an arched trellis over it.If so we'll also hang an urn shaped basket from the middle top inside and roses on the outside.I have heard red salvia and dusty miller are a pretty combination.I can't think of the name of the grass but its burgundy with brown plumes and it looks really nice against red brick if your box is brick.
I have seen zebra grass also really tall that looks nice at the base of a box.Also dark almost black looking iris catch the eye.Purple Hyacinth Bean Vine looks amazing.So does the white version.I bought some seeds this year of blushing susan vine and am thinking about that also.I have seen all these ideas.I always make a mental note when I see a mailbox that catches my eye :).One of the most memorable was Nora Barlow Columbine.It was unexpected and REALLY stately looking! Lowes has all kinds of columbines and clematis.Went yesterday.They had some really nice peony bushes with blooms on them too.Hope this helps.

I have a thing for planting scented items near the mailbox so that we, the mailman, anyone who comes by to drop off the neighborhood newsletter, or anyone just out on a walk gets to smell the wonderful smells. I have had great luck with all sorts of herbs (majoram, rosemary, thyme, sage) and mints (purple basil, bee balm.) I also planted tulips there with great success - they bloom when it's still cool, and seem to like the dry heat of being near the pavement during the summer hibernation. Irises like the dryness, too, as do daises. I try not to plant anything near the mailbox that is fragile, as it is prone to being abused (mailman gets too close, etc.) I plant the tall things in the middle, then grow shorter things in the back so I have a nice view from the house, too!