SimpleLogic Gates is a mod which adds redstone gates, inspired by the designs found in RedPower 2 and its spiritual successors, in particular RedLogic.

Currently, the following gates are provided:

OR/NOR, AND/NAND, XOR/XNOR
Multiplexer
Pulse Former
Buffer (0.5.0.194+), Inverted Buffer (effectively a NOT gate)
Set-Reset Latch (0.5.0.194+)
Randomizer (0.5.0.194+)
Synchronizer (0.5.0.196+)
Bundled Inverter (0.5.4.0+)
Bundled Transceiver (0.5.4.0+)
Comparator (0.5.4.8+)
Timer (0.5.4.8+)
Bundled Transposer (0.5.5.0+)
Repeater (0.5.5.0+)

Additional functionality is also available:

Shift-scrolling lets you switch between the gates you have in your inventory without opening the inventory menu!
You can invert any input or output of a gate by putting or removing a redstone torch on it.
As of 0.5.0.248a, it plays well with Redstone Paste!

Recommended mods:

Works best with SimpleLogic Wires!
Also works well with Redstone Paste.
If you're doing complex circuits, you may find Charset Tools's Signal Meter and Stopwatch really handy!

Behaviour differences from typical implementations:

Certain gates (such as the Multiplexer, Synchronizer and Pulse Former) cooperate with analog I/O.
Randomizers will not generate random values continuously - you have to cycle them externally.
Randomizers, upon receiving a weak redstone signal, will generate random analog values as opposed to digital ones.
Set-Reset Latches are actually JK latches - the difference is that triggering both inputs at once will cause the value to toggle, instead of unpredictable behaviour.

Known issues: