PPP and SLIP are integral parts of Linux. Their support and the number of channels supported are configuration options when you build the Linux kernel. Usenet news and Internet mail are also included. The software to support news includes the standard systems available on Unix platforms. INN seems to be the most popular.
Mail is handled by sendmail for most systems.
While not as capable, smail is also available and may be a better fit for low-end configurations. Various web servers are included with Linux. At SSC we chose to use Apache because it is reliable and efficient. The fact that we handle about 100,000 hits per day on a 486 system with 16MB of RAM tends to support our choice. For those needing a secure web server, they are not free but are available.