I already wrote a post about TWikiStandAlone project, but after I read the minutes from GeorgetownReleaseMeeting2008x03x03, I realized many people could be thinking "Cool, but what is the benefit for me?". So I'll try to answer this question.
The standard way to run TWiki is as a CGI script. It's easy to setup and many web servers and hosting services support it. But it's also slow and resource-hungry. There are some hacks and discussions about using things like ModPerl and PersistentPerl (aka SpeedyCGI), but I don't think they are that easy to use currently and some stability problems may appear.
The initial proposal of TWikiStandAlone project, as name says, was to turn TWiki into a standalone server, so that could be performance benefits and it would be easy to use, but things changed a little: in order to make it possible to keep using TWiki as a CGI script I achieved an architecture design that makes it possible not only to run TWiki as a CGI script and as a standalone server, but also in many other ways, including ModPerl and FastCGI.
Then, I can point some benefits:
Use of mechanisms other than CGI is easy
No more ugly hacks to use ModPerl
No patches needed to use FastCGI
Use as CGI and CLI is exactly the same, I mean, it's totally transparent
Standalone execution turns it very simple for personal use (No need to install/configure/run a web server)
Both FastCGI and Standalone execution are really fast compared to plain CGI
It's easy to take advantage of FastCGI and/or ModPerl support on many hosting services
It's easy to predict how much RAM will be used by FastCGI or Standalone execution mechanisms, so it's also easy to do capacity planning
Standalone execution permits to run TWiki as a different user than web server
In addition to all this, it opens some new possibilities:
Persistent execution can be even faster if tasks common to all requests were performed only once
Memory usage can be decreased by sharing common objects between TWiki processes
Persistent execution with DatabaseStore can help a lot with scalability. It would even be possible to easily build a load-balance cluster of TWiki installations
Standalone execution could permit execution inside some jail/chroot environment, decreasing the impact of possible security holes
If you think that none of those benefits are interesting for you, at least it's not harmful.
Up to now, development took place at it's own branch, but it is in process to be merged to core. I invite every one, specially developers, to read TWikiStandAlone topic carefully, more specifically implementation details and changes sections.
Feel free and invited to ask questions, make suggestions and get involved with this project!