SOPC Builder tool without NIOS processor

It is definitely possible to use SOPC builder without a NIOS processor. However, SOPC builder will force you to connect all Avalon slaves to a master. For example, the SPI, I2C, etc components have a slave port which must be connected to something.

It is possible to create a custom component which is an Avalon master which simply exports all the Avalon signals. Through these signals, you can then control your SOPC system directly from your VHDL code.

In many cases just using a NIOS processor can be easier. In the past licensing costs could be an issue, but in later versions of Quartus (starting with version 9?), the NIOS II/e core is free, even in the web edition.