I was wondering if there's some sort of online anatomy creator or maker? I'm wanting to make a model of a modified human body... is there some sort of anatomy creator that allows you to add organs or perhaps just an app or something. Or something that allows you to just make a new organ entirely? (if there isn't it's fine I'm just wondering so I don't have to draw out and design an entire body system... again...)
As indicated in my comments, it's not at all clear what you're expecting to get out of this software.
I highly doubt anything designed specifically for what you want to do exists. You're asking about an extremely niche application that very few people care about.
If, on the other hand, you just want something that's an improvement over drawing anatomy diagrams with a pen(cil) and paper, I would strongly encourage you to take a look at Inkscape.
Inkscape is a vector drawing program that is 100% free in both senses (free beer, free speech). Unlike traditional raster paint programs (MS paint and similar ilk, GIMP, Photoshop), in Inkscape you work with shapes, not pixels. This means you can easily draw individual parts as separate objects and it is trivial to transform (rotate and move) them or change their layering as much as you want. It is very nearly as easy to change their shape. Vector graphics also have a major advantage over raster graphics in that they are resolution independent. You can zoom in (almost¹) infinitely and can make unlimited changes with (almost²) no loss of quality.
The down-side of vector graphics is that it can be very difficult to impossible to achieve a "photo-realistic" look. However, this usually isn't a problem if you're dealing mainly with representational drawings, for which the trivially-obtained "super clean" look of vector graphics can be an advantage.
Also, while I'm not aware offhand of any SVG drawings of human anatomy, I wouldn't be surprised if they exist, and because of the nature of vector graphics versus raster, such a drawing will be far easier to alter to taste than a comparable raster image. Even if you have to make something from scratch, well, at least next time you'll have that head start.
I have not done what you are attempting to do. However, I have used Inkscape to do mock-ups of external anatomy (read: body shapes of both humans and non-humans). For example, see this answer; specifically, this picture, which was made in Inkscape. This isn't the greatest example because for that instance I only needed a very rough mock-up of a humanoid body. However, I've done similar drawings that included "articulation skeletons" that allow me to pose the figures. You can see in that example how I've used this as a guide for modeling furniture that is correctly proportioned.
(¹ At some point, you'll hit numeric and/or memory limits, but did you really need to zoom in to a scale of 1,000:1?)
(² In reality, certain operations can accumulate precision errors, but in practice you're unlikely to notice unless you edit an object thousands or millions of times. Also, you can avoid this by careful use of explicit transformations and, if necessary, manual editing of the underlying SVG data.)
This is not an anatomy editor specifically, but Photoshop, Gimp, or any other layer based graphics software may be able to serve your purposes. Start with any anatomy diagram you want to use as your baseline. Using the polygonal lasso, cut and paste all of the organs into thier own layers. Finally make a layer that is just a silhouette or non-organ populated image of the body for the background. With each organ on it's own layer you can rotate, clone, overlap, resize, etc. each organ separately as needed.
I would suggest trying to start off with a high resolution diagram since you tend to lose a little bit of resolution if you try rotating or enlarging layers.
Another option as Matthew points out is to use vector graphics. I do not know know enough about Inkscape to know if this is an option in that program, but in Illustrator, you can import a diagram to multiple layers and vector mask each layer to show just one organ each. This will give you the higher quality and ease of starting from a pre-existing anatomy diagram, but the vector software will prevent pixel lose as your images rotate and scale.
It will be a bit of work to set up, but once you have the layer separated original file, you will be able to reuse it over and over again without having to redraw things any more.