Beginning in late 1996 or early 1997, a new style of porting became available on full-size and compact Glocks. Different from either of the previous styles, it consisted of two ports side-by-side at approximately 11 o'clock and 1 o'clock positions on the top of the slide/barrel assembly. These ports are oval-shaped, elongated front-to-rear, and the corresponding cuts in the slide are considerably longer than the ports, allowing gas to continue to vent through the ports even as the slide unlocks and begins to travel to the rear. The V-shaped offset position of the ports prevents the exiting gasses and any associated flash from affecting the sight picture during firing in low light; in fact, most visible flash occurs above the slight plane, and is far more visible when viewed from the side vs. directly behind the ports, where the shooter is located. This type of porting has been used on Gen2 and Gen3 Glocks, and is currently available only on Gen3 models; as of this date, no ported Gen4 models have been seen or announced.
EDIT: In late May 2014, there was a report (with photos) of a short run of ported Gen4 Glocks for the European market. This was only a few models, and when someone asked Glock USA about it, they said it was a limited run, with no plans for expansion or any similar offerings here in the US. The photos of the Gen4 Glocks showed the V-type of porting seen in this post.
Ported Glock 19C and 17C models (G19C has Robar's NP3 aftermarket/custom finish on slide and barrel):
Close-up of dual V-positioned ports: