For homebrew it can be anything. I change things up pretty frequently so I wouldn't say I have a beer that I always make in the summer. Variations of wheat beers are pretty common, as are hoppy beers. For commercial beer it's Abita Strawberry Harvest Lager. By the summertime it's usually long-past being sold out, so I try to hold on to them for as long as I can.
And of you haven't made a patersbier, I'd highly recommend it for the summer. Pilsner up to about 1.045, mashed at 149, light hops, and - here's the cool bit - fermented with Trappist yeast.
I did Northern Brewer's recipe kit with Wyeast 1214 and it was pretty spectacular for being such a simple recipe. Perfect summer sipper.
I'm a pretty big fan of berliner weisse in the summer. Wish there were more commercial examples. Golden Road makes an excellent one, but they don't can it.
Commercially, anything from a pale ale to a hefe. So far as home brew goes, this year it's my Dampfbier & my 1st saison. Also have an ESB & a red ale for BBQ's.