Hello there. Welcome to the mechanism library!
Disclamer: I don't know who some of the original mechs were made by!!
~Arms and Retrievers~11952870 from RobotmafiaExample solve:
Halfmaster1This is a very basic retriever, goes back and forth 90 degrees.
11298502 from txsbor25Example solve:
txsbor25A two segment retriever, it has proved to be very useful.
11299066 from WafflekinsA very useful back and forth retriever.
11301806 from ProtogeniousVery cool mech, the "reflex quadrilateral". Turns approximately 90 degrees then back 180 degrees.
11746646 from FossExample solve:
FossThe amazing and wonderful, 570 degree brown mech :D
11795963 from PawelA great 180 to 360 degree turning arm.
11952917 from Ken75Example solve:
Ken75Super useful arm that orbits the build area, loads of room for modification
11849529 from FossExample solve:
FossAn amaaaaaaaaaaaaaaaaaaaaazing flippy floppity blippity boo. May be hard to find other uses for this mech.
11954687 from Ken75Example solve:
Ken75An arm that goes all the way around the build area
11994666 from DmastersExample solves:
Dmasters Dmasters againA super cool moving thrower thing
~Catapults~ (There's a lot of them, we need more on here!)
11952922 from Ken75 Example solve:
Ken75A basic brown pult, pretty poorly made :P
11953417 from {???}Example solve:
{???}Super useful brown pult for vertically challenged build areas
12009520 from MarjoExample solve:
MarjoA basic pult mechanism
~Bridges~11953425 from FossExample solve:
FossFoss himself has shown that this underbridge/pult is super uselful
11973122 10860484 from Rianbay May make a dissection so you can see the mech more clearly if it's requested
11976810 from Ken75Example solve:
Ken75A very easy to tweak and long reaching bridge, best on levels with tall build areas
~Miscellaneous~11401958 from RianbayA way to join two bridges under one weight.
11865925 from Camero09Example solve:
Ken75A mech that moves an arm back and forth 4 (or maybe more) times!
11952889 from RianbayExample solve:
Ken75 (Middle part has one of them)
One of the single most useful mechanisms ever, locks in place so things can be stable with no weight.
Also, check out the tutorials section in the Contraptioneering Videos thread, there's a few more complicated things that are shown step by step.