I tried to compile a list of the apps mentioned above, some of the apps I had a look at before downloading and saw that reviews weren't exactly spectacular (incl. picplz), so I didn't download them but these are the ones that I have downloaded:
2. Vignette Demo
I ranked them in order of most to least liked. I think both mytubo and Vignette are both very good however, mytubo is much more user friendly and great for people who just want to take nice photos without fiddling around with too many settings. Mytubo has 18 different effects and some basic editing options (e.g crop, border...etc). The photos automatically have their corner's taken off, like that rounded corner effect you see with Instagram. It has adequate online photo sharing options i.e. you can share your photos on the most common online communities i.e. FB, Twitter, Tumblr, Flickr.
Vignette is also very good and you can share your photos on pretty much every app you have on your phone that has a photosharing function (which is a plus over mytubo if you want to share photos through the communities not mentioned above). It has A LOT of settings and for those of you who like tweaking photos then this is probably better for you...although I have to say, it's actually a little confusing to use but I think you will get the hang of it if you play around with it.
Lightbox has a pretty simple layout and like mytubo it also has 18 different effects and basic editing options. One massive negative I have to mention though, is that when you take a photo there is a slight lag time from when you tap the screen and when the photo is taken...so you need to hold the camera still (it actually looks like it takes a photo initially because the screen freezes on the image but you need to hold the phone still for another second or so when it actually takes the photos...took me a little while to realise this; kept getting blurry pics).
Anyway, this is just my view, so I highly recommend you download these apps and try them for yourself to see which you like best (you can always delete them after if you don't like them).
However, since this is a thread for an instagram equivalent, I would say that the photos taken with mytubo look the most like Instagram photos. The effects options seem quite similar to the polaroid-like, vintage look of Instagram photos plus the automatic rounded corners of the photo is similarly also very like Instagram.
Hope that helps!