Not everyone has an IRC client installed on their computer, and for many people that only use IRC occasionally, or may be one-off visitors to IRC, it’s useful to provide alternative access, usually in a form that can be accessed via a web browser.
Java & Flash Clients
Java and Flash both have the potential to connect directly to an IRC server without the web server acting as a middleman. They can be embedded into a webpage, but, unlike gateways, once downloaded, they don’t consume server resources, and they all make seperate connections to IRC servers.
Mibbit is a large, public web gateway designed to be easily embedded as a widget on other pages, as well as used directly. Mibbit is able to connect to most networks, and when used directly, supports multiple network connections and most features of a desktop client, but because it proxies connections to the IRC servers through it’s own service, it’s been banned from some servers and networks at various times.
Other Public Web Gateways
Various public web gateways may be usable for your purposes as well, but are generally associated with or restricted to specific networks.