Off the top of my head, and in no particular order:
1. Secrets of the Iron Titan
Those first three all have some degree of overlap with each other, but are entirely compatible.
Ultimate Psionics contains both a construct race, and a class suitable for construct characters (the Aegis).
The Tinkerer series.
I've heard good things about the Machinesmith also, but I don't have it so I can't really vouch for it.
As Deranged_Maniac_Ben said, the just-released Advanced Races Compendium from Kobold Press has an entire (greatly expanded and revised) chapter on a construct race - the Gearforged)!
Full game mechanics, background and history, class archetypes ... the works :)
I'd definitely recommend giving it a gander!
So, just a quick question, do any of these products let you say, use other forms, like centaur or quadruped? One difficulty I have with a lot of construct options is that they generally only allow you to play a humanoid character-- granted, I understand that in part that's due to balance concerns, but I'd like something that gives the option to go beyond human form.
In fairness, the game generally assumes you're playing a human-shaped character. XD Outside of the options already mentioned here, you could also talk to your GM about templates. Or use the Race Builder and make your own.
Part of it depends on the kind of construct you want to play, really. Do you want to be what's basically an artificially intelligent robot, a more mindless sort of machine that obeys someone else in the party, or...?
Honestly, this is for a 3pp setting that I'm hoping to eventualy put out, but since I prefer to use pre-existing rules as much as possible, I was looking for already existing stuff. Mainly, I was thinking of something more like an alchemical from exalted in concept-- something that can be configured to a vareity of forms, from human to very much not.
My personal favorite is Fabricants:
They are based on any of the core races but as constructs, and then can choose to either advance as any class, take their own base class which allows them to progressively mod themselves with cool devices, or combine the two with feats and multiclassing.
They get really cool stuff, some of which cant really or easily be duplicated elsewhere. For example, they get inbuilt weapons, and at maximum power these weapons can be used for cool special moves: the flail can be used to attack in lines or cones, the drill can tear armor off opponents or cause earthquakes, the axe can chop through any material, etc.
There is also a prestige class that allows non fabricants to get some of their upgrades, cyborging themselves a bit (Fabricants who want to be spellcasters are not precluded from taking it either)
It does feel very similar to the Alchemicals, though they cannot change their modifications on a daily basis (and of course, they never become cities...)
So, does anyone have a good list of 3rd PP for construct races?
The main thing I'm looking for are products that don't force you to be always chained to a humanoid body, rather than having a certain freedom to build as one chooses.
Not being chained to a humanoid body... Okay, this is brilliant and you have my word that I'll be addressing this come Ultimate Tinkering time, maybe before if inspiration hits.