I'm trying to convince the company I work for to contribute towards open-source software, specifically building a library/component for ASP.NET web apps. We have an 'Innovation Day' coming up where we can work on our own personal pet projects at work, similar to Google's one day a week policy, except ours is once a month :-), and I have an idea that I want to open-source.
Other companies do this, eg. Headspring Systems (http://automapper.codeplex.com - see banner at top).
What do I tell them? What benefits can I convey to them that would be beneficial to the company? I've already mentioned possible exposure for our company and reputation etc. as well as attracting top software developers when we next go on a hiring spree. But what other arguments could I make?
Update: The company I work for is a software company that mainly builds web applications in ASP.NET and the MS Stack. Our clients are mainly the NHS (public health sector of the UK).