Many effects are based on a tiny delay applied to the sound signal and feeding it back the original signal and so on.
The best known of such effects are the phaser and flanger.

Reverb is the signature of the space in which music is being played or heard.
Recordings are often made in a space with as little reverb as possibly, so that the sound can be optimally recorded. The end result as is would sound unnatural, because we are used to hearing sound in a space (even outdoors is a space!), and in the majority of cases reverb is added in the final mix to create the illusion of space. Reverb is also a useful tool to extend a sound that stops short.

In olden days, a large empty space, preferably with stone walls (such as a church), might be used as a reverb 'device' by simply playing the sound through a speaker in the space and then recording the result. For many years, the Finnish Broadcasting Company had a reverb room, a large space with concrete walls and convex plexiglass reflectors hanging from the ceiling. This created maximum reflection to disperse the sound during recording.
Other early reverb devices included the spring reverb and the plate reverb. A spring reverb was a device where sound vibrations were conveyed into one end of a spring and recorded at the other end, producing a metallic timbre. A softer sound was produced by a plate reverb, where sound vibrations were conveyed into one corner of a large thin metal plate suspended in a box and recorded at the opposite corner.

Today, we use a wide variety of software-based effect plugins to emulate a space based on its physical data (room size, volume, shape, initial reflections, etc.). The most recent development in this field is with Impulse Response (IR) reverb, where the reverb in the desired space is recorded and the IR module is able to apply that reverb to other sounds. Apple’s LogicPro, for instance, contains the Space Designer plug-in that provides reverb emulating spaces of various types and sizes. It also includes a tool to make your own IR's.
IR produces the most natural-sounding illusion of a space. There are plenty of IR reverb files available online (example).

Examples of possibilities that IR reverb enables:
- Alice in Wonderland, dialogue recorded inside a teapot
- The Taj Mahal mausoleum is difficult to get to, but its reverb is available and is really great
- in cinema, dialogue recordings with disruptions are fixed by recording an IR from the original location and then applying it on a dry (no reverb) dialogue
- some organization could launch a project to record the acoustics of Finnish wood churches to create a repository of Finnish audio-cultural history (before the Churches burn down)
- if you want to remove reverb (normally a very difficult process), you can record the IR of the recording place and add it to the original recording with inverted phase

Here are some reverbs... (coming soon)

Delay does exactly what it says: it delays the sound. The end result is like repeating the same sound over and over again, getting quieter all the time.
One of the greatest advantages of the delay effect is that it can be set to repeat at a specific tempo.

More on Wikipedia

The arpeggiator applies an arpeggio, i.e. modifies a static chord in predetermined ways. This effect only works with MIDI data.
The arpeggio can be directed upwards, downwards or to repeat notes in the chord in a specific or random order.

Article about Arpeggiator

Various arpeggiators are used at the beginning of my piece "MiniMal".

More descriptions coming later (Chorus, Limiter, Compressor, Expander etc.).