There sadly aren't many like this, but I quite enjoy the ones that work as a nice, ambiguous shirt regardless, but might have a symbol or a logo that only another fan of the series would recognise. I think Treehouse Brand Stores might have now turned to ThinkGeek, but they run the Bethesda/Bioware/Borderlands/Tomb Raider stores, and there are some genuinely stylish shirts/hoodies for both genders. If you're going to go with the out and proud gamer shirt, Fangamer's are just beautiful. I don't have any myself, but I have a desperate need for all of their Earthbound shirts.
I do like a good gaming ringtone, too. Hearing the Fallout 3 level-up tone as a text message in public is always a nice surprise. I nearly fell onto the train platform in shock, but again, nice surprise.