. . . even if they're a bit odd.
There are dozens and dozens of monsters on my "use frequently" list. But these four are kind of odd standouts, to me - ones you wouldn't call "standard, common D&D monsters" but I used them often anyway.
Norkers - I love Norkers. I'm not sure what it is about armored hobgoblins with a bite attack, but ever since WG4 I've used them often. A couple Dungeon magazine adventures featuring them put them front-and-center in my early GURPS games, too, and they never left the spotlight. They've appeared in my current game, too, and I even bought like 10 norker pre-painted minis to add some fun to encountering them.
Yeah, I don't know, but I've used them often and I really like them.
Vilstrak - The first time one of these popped up for me was in a UK-series adventure I used in an AD&D game to introduce a 1st level monk. The next time I was that adventure was in GURPS, and so I converted the vilstrak to GURPS. And I kept doing so. My previous campaign group missed them, but my DF have fought "rock trolls" based on them, using some vilstrak minis I got when I cleaned some store out of their old AD&D miniatures blisters. I can't tell you quite why I like them, but I do.
Vegepygmies - I loved these guys in AD&D, and I used them there and I used them often in GURPS, too - very memorably for one group, where vegepgymies and thornies wreaked havoc on them. Killer plants are nice, but tend to be limited. Killer plant men, though, are sort of the best of both worlds!
Meazels - I've used these in GURPS and AD&D multiple times. I even drew on my love of them (and those double-creepy boogiemen from Babes in Toyland) when I wrote up the Bugbear for GURPS Dungeon Fantasy Monsters 1. I always ignore the "Number Appearing: 1" because garotting murdering stealth monsters are better in groups, just like in that Russ picture from the Fiend Folio. My current DF game is without one so far, but I did use its spiritual brother, the bugbear, so that counts for me.
How about you? What somewhat unusual monsters do you break out all the time?