Leopard geckos, for all intensive purposes, seems like the best all around "pet" lizard. You could, as noted, manage a pair of leopards in a 33gal, horizontally-based aquarium.
Pictus geckos are fun and hardy lil guys, but hard to come by in the US (or least hard for me to come by). Careful with mixing these guys tho, as males can be aggressive.
Ive mixed a number of species and have had results to show why NOT to do it and why TO do it.
I had a juvie Crested and baby gold dust day living together with a Pacific tree frog- that was interesting. No real combat, except the ONE time the crested thought the day geckos tail was a worm and CHOMP... bye bye tail. Grew back fine tho and the cresty never took another shot at the lil guy again.
I had a community tank with 2.3 anoles, a large male bahama anole, a western fence lizard (for a VERY short time), a ribbon snake, 2.2 fire belly toads, a fire salamander, 4 pacific tree frogs (no idea on sex but got breeding calls, courtship behavior and eggs from both these and the fire bellys), the pictus gecko, 2 house geckos, anywhere from 2-10 dojo eels (depending on how many the salamander and snake ate- was AWESOME to watch this btw) AND 3 mollys (fish).
Well lets see, where to start. The salamander, the snake and the pictus would sleep on each other- no joke here, ON each other. Dunno how that worked out, but i witnessed it on at least 3 seperate occasions- freaked me right the hell out.
The snake was continually hunting the tree frogs, but never tried to attack. Just followed em around the cage all day, which the frogs eventually got used to and some would sit on the snake while it watched others.The snake also refused to eat the orange mollys, yet didnt mind taking the dojo eels and small danios id place in. He DID eat the frog eggs, tadpoles and froglets though, which was pretty crazy to watch(not the toadlets, strangely enough).
I tried to introduce my large, fuzzy-mouse-eatting California Toad into this equation... the result? The first critter to try to check him out was the ribbon snake, who was swiftly toungue slapped and sucked head first into the massive toad maw! If I hadnt been there to grab his lil tail and give him a tug to pop him out, and remove the toad, that toad would have eatten that snake- no question in my mind.
The anoles battled constantly. The grizzled vet, named Wasabi, was a beast and ran the whole community- when he head bobbed, EVERYONE paid notice. The only animal he wouldnt whoop into line was the salamander, which would watch him, then crawl back down into the rocks. The anole males battled the bahama anole. Both of which took a try at the pictus, by the name of Chico, who womped their asses back into the branches with their dewlaps between their legs, lol.
Yet nobody died, no serious quarrels even. Everyone seemed to thrive- when crickets would be introduced, there was no fighting- everyone got what they needed. The anoles took those that came into the branches (canopy of local bambo, manzanita and a live bromeliad which ended up housing some of the baby tree frogs) with the help of the bahama, the house geckos and the tree frogs, while the snake, sal, fire bellies and pictus picked off the ground runners.
These were all housed in a 65 gallon Reptarium with a large filtered water area, tons of canopy space, full spectrum lighting (mostly for the plants) access to natural sunlight in the mornings (to the joy of my anoles!), and more hides than one could shake a stick at- moist rocks piles, dry rock piles, cork bark, some kinda weird local bark that i had autoclaived (sp?), a hollow stump, a large area of bed-a-beast, an area of dirt, an area of moist moss (fed by overflow from the water feature), a ton of random leaf litter (mix of dried bamboo husks/leaves, dead bromeliad leaves and the remains of a dried up fern), etc.
It was an amazing cage, but just too much work. My friend took over most of the residents, with the tree frogs being re-released into the wild MUCH fatter for their time with me 
Sorry to jack ur thread there... got alil nostalgic....
Anyway, personally, Id say try for pictus if you can. They are relatively small, very cool to look at, and not worked with as commonly as Leos. There are some cool morphs of these as well, though, as I said, harder to come by.
Other species I might suggest (im not 100% on how big a 33 gal is, so im gonna guess real small, lol):
-Anoles (these are AWESOME animals if you want something interesting to watch- get a group and watch the drama begin! lol...not to mention, these are some of the most common lizards in pet shops, yet I dont think ive ever personally seen any interesting morphs- no albinos, no melanistics, nothing... could try your luck)
-bahama anoles (slightly larger and not as colorful, but still very cool)
-salamander species (seems easy to keep humidity up in such a small space and these guys dont need a ton of room... pretty interesting to watch too- smarter than youd think)
-tree frogs (if you do it right, you dont even have to turn your cage on its side... people think arboreal species and they think, oh, it has to be tall... but if you minimize substrate (not a prob with most arboreal species) it gives you quite a bit of room to make use of what vertical space there is and with proper setup, can easily house a tree loving species)
-day geckos (these guys stay quite small, usually under 5" i THINK, and are almost strictly arboreal- never saw my gold dust day touch the ground in all the time i had him... a certain kind of tree, I know they are called "money trees" work GREAT for these guys, providing both climbing structures AND hide spots- just great, especially with minimal space)
-obscure lizard species: African clawed geckos were on sale from LLLreptile awhile back and BOY where they incredible to look at; though they are apperantly rather hard to care for.... Red eyed armored skinks are INTENSE animals, look like dinosaurs, but have difficult humidity requirements i think.
There are TONS of species to choose from man, dont have to just limit yourself to geckos. Hell, despite what many say, Im a believer in the idea that 2 species can easily co-exist. Its just a matter of having the right animals and providing them enough resources to not force a confrontation- as long as they dont become territorial, you shouldnt have any problems.
Pictured Below: Chico the Pictus Gecko- Male, 8-10 months old
