Bluntly: not many. There are no software alternatives to the MSM available that I know of. Further if they did exist, I strongly doubt that they would be compatible with the core EE license, specifically:
An ExpressionEngine license purchase allows a single “live” installation in a production environment and ancillary “development use only” installations as needed to support the live installation (such as development and a staging servers). Additional installations require additional license purchases. If you have specific installation needs not covered by this license, please Contact Us.
Furthermore the license also states:
Unless you have been granted prior, written consent from EllisLab, Inc., you may not:
- Reproduce, distribute, or transfer the Software, or portions thereof, to any third party.
- Sell, rent, lease, assign, or sublet the Software or portions thereof.
- Grant rights to any other person.
The only extension/exception to this is if you purchase the MSM.
However, if you are willing to purchase a separate EE license for each of your sites, but want to share template code between them, you could easily manage that at the file level, either via symlinks (if all sites on the same server) or via site specific branches in a Git repository. Though that doesn't resolve the shared schema/fields/channels issue.
There are a number of experimental addons/products that seek to address that issue in varying ways (eg NSM Site Generator & Site Manager), but none (that I'm aware of) that are production ready.
Lastly. in my experience EE is not the best choice for a SaaS, multi-instance application for precisely the reason that channels and fields are implemented as DB data not as part of the schema. That makes for great flexibility and rapid development, but it also makes sharing site structure across installs much more difficult that with a lower level framework.