There really isn't anything available that can physiologically increase penis size or girth. However, there are other options. Oxballs is a sex toy company whose target clientele are (kinky) gay men. Among the products they sell (check out the rest here - NSFW) are penis extender sheaths. This is what they look like:
They're worn over top of the penis, adding length and girth (see the rest of the NSFW photos here).
Oxballs isn't the only game in town - other companies are making similar products. If this sort of product could be marketed to straight men in a way that doesn't bruise their egos, I could see something like this becoming very popular for those men looking to gain a few centimeters (or more). In fact, some have already started to dabble, according to sex toy stores.
Dan Savage addressed penis sheaths in a Savage Love last year: link.