It's a windows environment application which makes it easier to use SVN. Normally you have to tap in all the commands, which is tedious. With Tortoise you just point and click.

I happen to use CVS, so I use Tortoise CVS

Tortoise SVN: http://tortoisesvn.tigris.org/
Tortoise CVS: http://www.tortoisecvs.org/

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.