Commonly-used links for Nonscriptum clients

Software:

CAD software:

Tinkercad (beginner drag and drop, cloud-based): http://www.tinkercad.com/
OpenSCAD(programming, good for math models, free download): http://www.openscad.org/
OnShape (more sophisticated engineering, cloud based): http://www.onshape.com/
Fusion360 (more sophisticated engineering, cloud-based): https://www.autodesk.com/products/fusion-360/overview/

Specialty software:

Printing terrain: http://jthatch.com/Terrain2STL/
Fixing problems with meshes: http://www.meshlab.net/

To reduce the size of an STL file:

Import mesh into meshlab.
Go to Filters > Remeshing, simplification and construction > Quadratic edge collapse decimation.
Cut the number of triangles by 50% (enter 0.5 in dialog box) as a first cut.

Slicing software:

Prusaslicer: https://www.prusa3d.com/en/page/prusaslicer_424/
Ultimaker Cura (alterable for some other manufacturers): https://ultimaker.com/en/products/ultimaker-cura-software/
Lulzbot version of Cura: https://www.lulzbot.com/cura/

Temperature settings, common materials:
Note: these are good places to start in the absence of other information. If your manufacturer and/or the filament manufacturer used something different, start with their suggestions.

PLA: Extruder temperature 210C. Bed temperature 60C.
If you have an unheated platform, print on blue tape or glue stick (never heat a platform with blue tape on it higher than about 30C).
PETG: Extruder temperature 220-260C, bed temperature 80-90C.
Higher temperatures typically produce a shinier appearance, lower more matte (note: you need a nozzle that can handle a temperature this high). Higher temperatures can give you better layer bonding, but you will also get more stringing.
If you have an unheated platform, print on blue tape or glue stick (never heat a platform with blue tape on it higher than about 30C). PETG may stick permanently to some PEI/Ultem surfaces. Glue stick can be used as a release layer to prevent this.
PETG should be kept dry, and benefits from using a filament dryer before printing.
ABS: Extruder temperature 230-260C (note: you need a nozzle that can handle a temperature this high).
Heated bed is essential. Bed temperature 110C. Enclosed printers are strongly recommended for ABS, and you should never use a print cooling fan (except inside a heated chamber).
TPU: Extruder temperature 210-250C (230C is a good starting point for most formulations). Bed temperature up to 50C.
TPU is extremely hygroscopic, and filament dryers are essential for good prints. TPU also needs to be printed slowly. Extruders without a well-enclosed filament path between the drive gear and the nozzle are not suitable for TPU, as it may buckle and go out the side.

Fixing a clog: search for “cold pull” or “atomic pull” for your 3D printer.

3D model repositories:

Book model repositories:

Other model repository sites: