I know some guys in South America, and for the most part there's not a lot of websites. Sao Paulo is the biggest scene I know of, but there's stuff in other Brazilian Cities, Santiago, Buenos Aires, and also Columbia (I haven't been to Columbia yet though). www.piratininga.org
is a Sao Paulo site, but it's mostly architecture/preservation focused. http://www.lost.art.br/zezao.htm
is another great site, but is basically a graffiti site. I haven't really found a pure exploring scene in any of the places I've been - the scenes are either mostly academic (architecture/preservation/urban planning based) with an exploring angle, or graffiti-based with an exploring angle. Both are fun, although some of the graffiti guys are seriously, seriously fucking nuts.
There's a lot of difference between exploring in large first-world, and less-than-first-world cities. For the most part, urban areas in non first-world cities have huge recent popluation growth due to immigrants fleeing poverty in rural areas. There isn't the abandonment due to urban population decline and deindustrialization that you see in North America or Europe, and as such there aren't the type of abandonments you'll find in post-industrial first-world cities at all. This isn't to say that there aren't dilapidated and abandoned buildings - they're just always filled with squatters, because there's so much housing pressure due to the exploding urban population. This is the biggest reason for the lack of photo sites - I dare anyone here to walk into a abandoned skyscraper in Sao Paulo with $3000 worth of camera equipment.
This poverty also leads to a lot of security. Any construction site (or anywhere else with metal) without security will be completely scrapped in 5 minutes. Any remotely middle-class neighborhood will have everything on lock-down. Any abandonment without squatters has a ton of security.
In terms of urban infrastructure, these are mostly recently developed cities. As a result, drains are almost all big and non-descript (and occasionally populated by squatters also). There isn't a whole lot of forgotten odds and ends from over a hundred years ago, because generally 95% of the city was developed in the last 100 years. There hasn't been any kind of real land war in urban areas, so you don't have any military abandonments.