Generally speaking there are no technical restrictions about what you can run on your VPS - almost anything that you can install on a dedicated server or a home PC will run normally on a VPS.
The below is a quick list of applications and software that you can run on your VPS:
- web servers: Apache, IIS, and nginx.
- web applications: WordPress, Magento, MediaWiki, Drupal, Joomla
- software development platforms: PHP, ASP.NET, Node.js, Ruby on Rails, Python
- database servers: MySQL, PostgreSQL, Microsoft SQL Server
- mail servers: qmail, postfix, Exchange
- groupware and communications: Exchange, Zimbra, Sharepoint.
- ...and many, many more.