A couple I know are Pterygoplichthys pardalis and Hypostomus plecostomus. They're now considered invasive species in the Philippines after being seen in hundreds by fishermen, not to mention rising incidence of torn nets.
Hi Silentskream-
I think Lupin's two suggestions are the two most commonly seen ones, but they're by no means the only ones sold as "common pleco" or just "plecostomus". The problem is that most of these are grown in aquaculture ponds in the far east, and even the owners probably don't know which ones they've got- they may have a mixture of species in the ponds.
If you take lots of pictures of the fish you've got you might be able to narrow it down by looking at fin ray counts etc.?
well the one i have is very dark in general. but his coloring is brown with black(or very dark brown) spots.
and he's got somewhere between 10 and 15 dorsal rays (i think.. hard to count when he won't show longer than a second.)
about 16 rays on his anal fin.
his underbelly is also spotted, not verticulated, and considerably paler than the rest of his body.