PHP

SMS-API - (Rating: 4.11/5 from Hotscripts.com)

This class sends SMS messages via the Clickatell gateway and provides support to authenticate to this service and also query the current account balance. This class uses the fopen or CURL module to communicate with the gateway via the Clickatell HTTP/S API. A full package is available including a test form, initlization script and latest sms-api class.
http://sourceforge.net/projects/sms-api/

Clickapi
This is a collection of classes for providing access to multiple Clickatell services using a web services API and very common helper libraries for ease of use. You can use either use the Clickatell XML API (over HTTP) or the Clickatell SMTP API.
http://www.phpclasses.org/browse/package/2014.html

Clickatell SMS PEAR package (Reusable PHP component)
This PEAR package provides SMS functionality and access to the Clickatell gateway via the Clickatell HTTP API.
http://www.powertrip.co.za/PEAR/SMS_Clickatell

Java

SMSJ- SMS library for the Java platform

This library allows you to send SMS (GSM) from the Java platform. It gives you full control over the SMS message including the UDH field so you can create and send EMS messages, WAP push messages and Nokia smart messages (picture, ringtone etc). The API can send SMS via a GSM phone connected to the serial port or via a SMS gateway like Clickatell.
http://smsj.sourceforge.net

Python

A python library for Clickatell is available here:
http://www.powertrip.co.za/code/python/clickatell.py

Perl

A perl module for Clickatell is available here:
http://search.cpan.org/~ralamosm/Net-SMS-Clickatell-0.05/lib/Net/SMS/Clickatell.pm

Ruby

A Ruby library for Clickatell is available here:
http://clickatell.rubyforge.org/

Applescript

SMS Sender Clickatell
http://forums.macosxhints.com/archive/index.php/t-26369

Joomla CMS

Mambotastic SMS Component
This lets you provide SMS functionality to your users , straight out of the box. The component runs on the back of the Clickatell gateway, and provides an interface for you to administrate and manage your SMS registration, subscription and marketing activities. Supporting bulk SMS, single SMS messaging with history, subscriber and configuration management and multi-language manager, you can be up and running with your own SMS engine for Joomla in minutes.
http://webscripts.softpedia.com/script/Modules/Joomla-Mambo-Modules/Mambotastic-SMS-1592.html

Moodle CMS SMS component

A block for sending SMS messages to users from Moodle (1.6,1.7 and others) via the Clickatell gateway using the SMTP API
http://moodle.org/file.php/5/moddata/forum/396/277091/sms_manager-20061219.zip

There is also a SMS library available at http://julmajanne.com/stuff/docs/smslib.html

Gordano Messaging Server script

This script enables Gordano Mobile Gateway Software users to connect via Clickatell's gateway
http://www.gordano.com/kb.htm?q=973