Question:
I do like Happy for men as well...and so does Chandler Burr.
Peace,
Otto

Answer:
Why, Annick Goutal's Eau d'Hadrien of course! That's one of the ultimate winners for me - in both the EDT and EDP. It is very real and refreshing.

Other citrus scents I enjoy and recommend during the summer are Bigarade Concentrée by Frederic Malle, Annick Goutal's Eau du Sud and Terre d'Hermès.

Answer:
Oh man, where to start. I love citrus.
Acqua di Parma and Happy I gladly second. But do not forget the divine Monsieur Balmain, Creed's Neroli Sauvage, Crown Spiced Limes, Capucci pour homme, and many more.
Edit: any good classical Eau de Cologne is a nice option, such as Roger&Gallet Extra Vieille, Guerlain's series of Colognes, Farina Gegenüber, if you can get it, Nicolai's Cologne Sologne, or the very nice MPG pour le jeune homme.

Answer:
Rochas Lui
Terre d'Hermes
Well they start with citrus anyway....

Answer:
It's not mentioned often in threads, but my favorite citrus is Iskander by Parfum d'Empire. Wonderful lemon/lime topnotes (and grapefruit, although it's much more subdued than I detect in other citrus scents such as AdG), coriander, and a nice rich drydown of musk and a hint of amber. I haven't worn it in hot weather yet, but I'm sure it will accomodate.

Answer:
Live Jazz
Trumper GFT

Answer:
Burberry's Weekend.
I like Happy for Men by Clinique but it doesn't last long enough on my skin.

Answer:
malle's bigarade concentree is great - a bit dirtier than what many people would call a "summer" scent. like i'm not dirty enough already
I'll also second the MPG Pour le Jeune Homme (a bottle will be arriving shortly), eau d'hadrien, and a couple of surprises: Clinique Tailoring for Men (if you can find the long discontinued gem) and Azzaro Orange Tonic (listed as femme, but distinctly uni IMO)

Answer:
Bvlgari pour homme extreme
D&G
fat lady's singing.

Answer:
There are so many good citrus fragrances out there.
Here are two I love:
Acqua di Parma Colonia
Annick Goutal Eau d'Hadrien