So you've made the decision to move away from your existing system and convert your whole project to Git. How can you do that easily?
Git comes with a script called git-svn that has a clone command that will import a subversion repository into a new git repository. There is also a free tool on the GitHub service that will do this for you.
$ git-svn clone http://my-project.googlecode.com/svn/trunk new-project
This will give you a new Git repository with all the history of the original Subversion repo. This takes a pretty good amount of time, generally, since it starts with version 1 and checks out and commits locally every single revision one by one.
In contrib/fast-import you will find the git-p4 script, which is a Python script that will import a Perforce repository for you.
$ ~/git.git/contrib/fast-import/git-p4 clone //depot/project/main@all myproject
These are other SCMs that listed high on the Git Survey, should find import docs for them. !!TODO!!
Mercurial (hg)
Bazaar-NG