A client-side application (such as a browser, a browser plug-in, a browser toolbar plug-in, etc. on an end user's computer) is used to support the serving of content-relevant ads to the client device. The client-side application may provide such support by sending document information (such as a document identifier, document content, content relevance information, etc.) to a content ad server. The client-side application may also be used to combine content of the document and the content-relevant ads. For example, the client-side application may combine content of the document and the ads in a window (e.g., in a browser window), may provide the ads in a window above, below, adjacent to a document window, may provide the ads in "chrome" of the browser, etc.