Is there data on how many users have Flash, Java, or Unity plugins installed?
This website extrapolates from a panel of some sort:
Bottom line as of posting this:
- Flash: 97%
- Silverlight: 70%
- Java: 75%
Others have mentioned Flash, Java and Silverlight. Sadly there are no official numbers for Unity I'm aware of. In a rather old blog post they talk about ~1% pre-installed user base. This article over at Gamasutra also assumes 1% penetration. Although the conversion seems to be rather high (people willing to install the plugin).
User base might also increase with gaming platforms like Kongregate supporting Unity Web-Games.
According to the Adobe Flash website, Flash Player Version Penetration, it's about 99-98% in most markets.
Java is also pretty ubiquitous. Unity is less frequently found, however the installer is inline and quite small in most cases.