selfoss is a multipurpose RSS reader, live stream, mashup, and aggregation Web application. You can register RSS feeds, and this Web-based PHP application will continuously fetch new RSS feed items. The items will be shown in a stream. You can also add other sources, like deviantart, Twitter, or tumblr users. Attaching new sources is very easy, and you can add any source you want (e.g. IMAP email account, log files, etc.). selfoss also allows you to collect all your postings on different communities (e.g. Twitter, your blog, etc.) and show it in one place. It features a Web-based RSS reader, universal aggregator, mobile support (Android, iOS, and iPad), and support for MySQL, SQLite, and MongoDB databases. It is easy extensible with an open plugin system (write your own data connectors). It is a lightweight PHP application taking up less than 2 MB.
Bluehorn Reader crawls RSS feeds and makes their content available to clients (only a browser client exists currently). It supports multiple users, whose sessions' look and feel can be customized based on each place they log in from. A potentially useful feature is that, while it should be able to support a large number of users from a single host, if that is not enough, more hosts can be added to share the load, with only minor configuration tweaks. The Web server part is mainly in JSP, so it should be easy to play with it and adapt it to support other use cases.