I've always had really good luck using Kobolds.
Now, before you laugh, consider a couple things.
1. Most players consider Kobolds nothing more than sword fodder for low-level parties and do not give them any sort of respect.
2. Kobolds are canonically described as "Cunning and vicious", with a predilection for traps and ambushes.
3. Most GM's do not play Kobolds anywhere near to their potential.
4. Given enough time, a Kobold tribe can dig out a massive warren of tunnels and chambers.
5. Kobolds breed and mature rapidly.
6. They are NOT stupid. Cowardly, and untrustworthy, yes. But the Average Kobold has an Intelligence of around 10, with the potential for much higher.
7. They are known to trade with other humanoid species, (or rogue human merchants) when they cannot steal or make something themselves.
Add all this up and you have the potential for a rather unpleasant time for the players. They think they are being hired to clear out a "Small nest of kobolds"....instead....
Their first indication of a problem is that the Kobolds they first kobolds they really meet, (after running a gauntlet of traps and hit-and-run ambushes) are better trained, equipped, and disciplined, than they expect. I usually use Roman Legion Auxiliary kit for their gear/tactics. (Chainmail shirt or boiled leather breastplate, shield, short sword, Spear or Pilum, and 4 Plumbata/Darts...poisoned of course) The only real magical items they'd have, would be the shields, which would have something along the lines of a spell-ward, which stacks when in close contact with another just like it. In other words, your mage tries to take the formation out with a fireball and watches as it dissipates harmlessly against the Kobold's Testudo formation. (Shield turtle)
Now you get a few waves of Plumbata/javelins to weather as you/they close. Granted, a plumbata tossed by a Kobold is only going to do a few points of damage, but there are a lot of them....all venom coated. Again, a low to mid-level venom, but best hope you don't miss too many of the saves.
Get past them and you have the warren itself to deal with. Miles of twisting tunnels, most of which are only just large enough for a Kobold to pass, certainly not wide/tall enough to swing a longsword. Oh, and did I mention the traps? Hidden doors/passages of course, as well as quite a few sliding walls that can, and will, be used to change the layout and/or trap the party. Indulge your most evil and twisted, sadistic impulses.
Oh yeah, don't forget the confines when calculating spell effects...especially volume affected. Mages tend to LOVE their flashy, big damage spells like fireball. Do that in a tunnel and see what happens >:D You get the same volume, but instead of a ball/dome, you start from point of detonation and start both ways up and BACK down the tunnel. Survived that? Best hope this isn't a part of the warren with shaky structure.
So, you get the idea.
Happy player torture!
