I'm looking for the following features:

1. Scheduler uses SQL Server as the backing store
2. Tasks can be scheduled by one application and executed by another
3. I can have multiple applications, each of which handles a specific subset of tasks
4. Tasks can be triggered at specific times, now, or based on the success or failure of other tasks
5. Data can be attached to tasks

There are a number of nice-to-have's, like a web management console, clustering/failover support, customizable logging, but they're not requirements.

On the surface Quartz.NET has a nice interface and seems to fit the bill as it satisfies (1), (4 with some custom work) and (5), but I've been beating my head against (2) and (3) seems like I'd have to invest more effort than it's worth, especially given how convoluted the innards of it are.

Any other libraries out there? Open source is preferred, with free as a close runner up. It's really hard to get management to pay for things like this when it's not their idea.
['https://Stackoverflow.com/questions/2815373', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/156767/']
I hate to say it, but it sounds like you want [BizTalk](http://www.microsoft.com/biztalk/en/us/default.aspx) which is neither open source, nor free, but is ideally suited towards what you are trying to accomplish.
You could use Windows Workflow (WF), maybe along with Quartz.NET