The STL Slicer works great...as long as you give it a "nice" STL it likes. You can slice-and-dice an STL at practically any angle/axis you want. View the tutorials at the Slicer website...the GUI isn't as bad as you might think...just have to know what the tool icons are for. The Tutorials will help with that. A brief 5-minute look-see won't do you any good as far as learning how it works.
It doesn't like STL with color/texture information, and it works best if the STL is 99MB or less.
There are a couple things you can do to assure your "problem STL's" will open in SLICER...and it's totally free to do so!
First, download and install the free program MeshLab. http://meshlab.sourceforge.net/
1) Start up MeshLab and create a "New Empty Project"
2a)... If you have a HUMONGOUS file, it is easy to reduce the file size without noticeable quality loss.
2b)...Import your STL file into your New Project in MeshLab and click "OK" in the Post-Open Processing window (see screenshot entitled "1-Post-Open Processing"
2c)...Select Filters/Remeshing,Simplification and Reconstruction/Quadric Edge Collapse Decimation (see screenshot entitled "2-Remesh_Quadric")
2d)...Take a look at the screenshot entitled "3-Remesh_Settings". Make SURE you check all the boxes exactly as shown in my screenshot. Now, type in a decimal fraction (percentage) of how much you want to diminish the mesh, without going too crazy, or you'll get visible facets on your model. NOTE: the decimal value you type in is NOT the percentage reduction... EXAMPLES: type .70 for a 30% reduction, type .50 for a 50% reduction, type .90 for a 10% reduction, etc. Also, MAKE SURE you have typed in a value of "1" for the Quality Threshold.
2e) Click the word "Apply", wait for it to finish, then click the word "Close" to get rid of the window.
3) Observe your model on-screen to confirm it is still good quality (it will be if you didn't "go crazy" with the remesh reduction)
4) Now, go to File/Export Mesh As. You will select your file type (STL in this case) and create a new name for your model (so you don't overwrite your original!). Click "Save" and you will see a new window pop-up. There is a setting here you MUST UNCHECK before saving to enable the SLICER to open your new STL (see screenshot entitled "4-Saving_Options"). UNCHECK the box that says "Materialize Color Encoding".
Click "OK" to save your new STL. It should now open in SLICER.
So, we have basically corrected the "troublesome" STL files by doing this. Even though many STL files will open in other programs (like MeshLab here) with the color or texture preserved, some programs won't open them or might even crash on you...the SLICER wants "clean" STL's without color info and a file size smaller than 99MB. Reducing the mesh to a reasonable size and discarding the color/texture info should put you on track.
I hope this has been helpful.
Additional Info: If you have a file that is already a reasonable size you are comfortable with, then you don't need to do the "remesh/reduction". However you WILL need to go ahead and re-save/export the model to remove the "Materialize Color Encoding" in the save options as described above.
An acquaintance mentioned importing, then re-saving from Blender (another free program) works well for "fixing" an STL. I suspect it is stripping out the material/color info by default. I'm not sure how good a job Blender does for reducing the mesh without a noticeable quality hit - maybe it's fine - but I know for sure that MeshLab does a great job with that.