The web shop where I work we do both .Net and PHP/Linux development. We'd like to start automating a lot more of our deployment processes using a common system that can be used for both. What would you recommend as a good common scripting language or automation system (like Ant or Maven) that works well for both .Net and Linux development?
You can develop .net on linux using Mono. On that linux server you can use both .net and php.
I have no experience with Maven, but we've managed to get Ant to do everything we've wanted on multiple platforms, just by virtue of the fact that you can extend it with any Java program to do special tasks not included with the Ant application itself.
And then Java can, if necessary, call any external program you want to by using Runtime.exec(). Loses a bit of the portability but it was required for some command-line tools under both Linux and Windows.
You may get some mileage out of Capistrano. It is rails centric but it is pretty general purpose, and I believe it has been extended to do other things also. Not sure how well it plays with Windows and .NET, but worth a look.
You may want to see how Perl does cross platform automation. The design for Perl6, seems to be a bit better designed, but the Perl5 design has had many years to develop.
Perl has been ported to many different platforms, not just Linux and Windows.