Taran Tactical, Dawson, and Taylor Freelance are the three that pop up most often. However, they are more expensive than they should be. You will pay $30+ for the extension and $20 for the Glock OEM magazine just to hold 23 rounds. ETS makes 22 rounders that are very solid for $20 or less. Magpul is also dropping their 140mm and 170mm magazines any day now that will hold 21 and 27 (I think) respectively for around $20.

Then you've always got the Glock 33 rounders, ETS 31 rounders, SGM 33 rounders, etc that will all come in below the cost of the +6 extensions.

The only reason to drop the money on the extensions is if you're planning on competing in USPSA because they have length restrictions on magazines. If you're looking for a range toy, or even something for a nightstand gun, I'd really recommend just getting an extended magazine instead of a magazine extension.