There are many tools around. If you use Eclipse, it can do XSD out of the box, perhaps rather basic, but it does the job and is free.
You can use oXygen, which is excellent, has good feedback, works stand-alone and with Eclipse and supports DTD, XSD, Schematron, Relax NG Compact and Relax NG (plus much more, but I don't want this to become a commercial ad). Oh, and it's rather cheap and with a very good helpdesk and support forum.
I understand you already know Altova XML Spy. It's an expensive, huge and bloated tool with many bugs. My experience with their feedback-to-bug-reports: they usually either deny it or just don't get back at you. I've had endless trials with them about their XSLT 2.0 two years ago, and blatant bugs were just kept in their products. In short: don't bother (but unfortunately, managers love the slick presentation of XML Spy and it may be hard to convince them otherwise).
Disclaimer: I'm not affiliated with Altova or oXygen in any way. I primarily use oXygen for XSLT 2.0 development and occasional XSD.