In other documents, most importantly in legal contracts, an addendum is an additional document not included in the main part of the contract which may contain additional terms, specifications, provisions, standard forms or other information. A contract addendum may also be called an appendix, an annex or a rider...

Addenda are often used in standard form contracts to make changes or add specific detail. For example, an addendum might be added to a contract to change a date or add details as to delivery of goods or pricing. The addendum should be referenced in the contract, or the contract should be referenced in the addendum, so that it is clear which contract the addendum is modifying.

A rider is often used to add specific detail and especially specific conditions to a standard contract such as an insurance contract. A rider may also be added to a piece of legislation.

Schedules and exhibits are sub-categories of addenda, with schedules being related to numerical and time information, such as pricing and time-schedules, and exhibits used for examples of standard forms and different types of evidence or models. Exhibits are often used in legal documents submitted to a court as part of judicial proceedings such as statements of claim and briefs.

http://en.wikipedia.org/wiki/Addendum

Supplements, Addenda в значении "Приложение" не встречаются никогда. Используются примерно так:

any amendments or addenda hereto;
no amendments, addenda, any other attachments to this Agreement;
The parties hereto may vary, amend, restate, supplement or substitute this deed;
as amended, supplemented or novated from time to time

Appendices & Annexes - практически никогда, опять же только так:

This Agreement (including any annexes, appendices or exhibits hereto);
All appendices and schedules to this Agreement, including any exhibits to the appendices and schedules.

Иногда бывает, конечно, Appendix 1, но ненамного чаще Annex 1 (по моим ощущениям).

Rider - опять же, в американской юридической практике - означает некий кусок договора, например, статью, или любой цельный piece of shite, который в него вставляется после первого драфта.

Еще видела такое: Any notice or another official communication (with the enclosure of a duly executed document evidencing the right...) - т.е. с приложением такого-то документа.