Cave of the Spiders for 9th level characters. Originally designed for D&D 3.5, the Cave could be played under the Pathfinder rules or another D20 system without too much trouble. The window of opportunity to pick this adventure up free has closed, but $0.99 will get you a copy of Cave of the Spiders.
I can't help but contrast Skip Williams' style with that of Owen K. C. Stephens. Cave of the Spiders has a straight up title and there's just one paragraph of background in the eight page document. I also can't help but murmur "Thank you!" for the straight to the point approach. Like a lot of the Original Adventures series, the Cave features a pretty full-colour map - probably from Map a Week, although it's not identified in the document. Unfortunately, the map is a cross-section and that means the referee will have to improvise when it comes to room dimensions. Those favouring tactical combat with models would be well-advised to make some notes while preparing to run the module. On the bright side, Williams was concerned by the site map not having a scale and gave instructions on how to achieve a 1 inch to 20 feet scale when printing. (Somewhat later in the production process, it seems that someone helpfully added scale to the map but didn't remove those instructions.)
Williams gives a few hooks for referees whose players "aren't in the habit of exploring every mysterious cave mouth they find". I duly skipped over them. But seriously, the hooks are OK if a little over-worked. This series would have been better if more of the writers had just gone with cash rewards to motivate the PCs. Everyone has a use for money, and referees who want to invent something more intricate have a better idea of their players' motivations than a module designer.
The adventure site is definitely coherent and everything makes sense, but it's hardly groundbreaking. Cave of the Spiders' real strength is the attention to detail in the key. Williams doesn't go in for a lot of variety of threats here, but he takes the time to explain how the monsters fight and respond to intruders. The majority of the treasure is in the hands of the Cave's defenders, though there is a fair haul for PCs bold enough to seize it.
My rating is 3/5, though I was tempted to pull for a 4. Relatively new referees, or those who struggle to come up with a defence plan for intelligent monsters encountered in their lair, should definitely take a look at Cave of Spiders. It would also serve well as a time-saver for other referees running D&D 3.5 or other D20 campaigns.