What kind of spider is this?

Top three contenders that are super common in Western PA (I live about 30 minutes from Pittsburgh):

Rabid wolf spider:

http://bugguide.net/node/view/40156

http://bugguide.net/node/view/26084

Nursery Web spider:

http://bugguide.net/node/view/2919

Grass spider:

http://bugguide.net/node/view/244810

Wolf spiders don't build webs, nursery web spiders aren't very common indoors (I've never seen one inside, anyway), and grass spiders both build webs and can be found in and against houses. ...So like John said, most likely grass spider.

All are totally harmless to you, though. For future encounters, please don't kill them! The only two dangerous spider we have in West PA are widow spiders- the Northern and Southern species.

I'd put my money on a Grass Spider, as there have been several questions and photos of these in the past couple days. They're funnel-web spiders that make a flattish sheet web with a funnel retreat at one end. The males are wandering around at the moment hoping to find a female, so they've been turning up in houses. The key field mark is the pair of long spinnerets at the end of the slim abdomen. Harmless, of course, like most spiders.

The size would indicate somehting from Pisauridae (nursery web or fishing spiders) or Lycosidae (wolf spiders).

While the description you gave is potentially a great number of spiders, including other families not listed above, it does sound quite close to:

Rabidosa rabida, the "Rabid Wolf Spider"

http://bugguide.net/node/view/26084/bgimage

Harmless of course. Your shorts are safe. In fact, this little fellow (if you hadn't killed it) could help you keep away pests that might actually cause a problem. In your shorts or elsewhere.

Another possibility would be a Nursery Web spider, which is the same family as Fishing Spiders (Pisauridae). Nursery, Fishing, and Wolf spiders are often misclassified. All three are common in Pennsylvania and all along the east coast.