Mirjam's blog lists some uses of SPContext.  Here are a few I grabbed from his post:

SPList currentList = SPContext.Current.List;
SPWeb currentSite = SPContext.Current.Web;
SPSite currentSiteCollection = SPContext.Current.Site;
SPWebApplication currentWebApplication = SPContext.Current.Site.WebApplication;
SPListItem item = (SPListItem)SPContext.Current.Item;
SPUser user = SPContext.Current.Web.CurrentUser;

He also explains the difference between the SPSiteDataQuery object and the SPQuery object.

Check it out here: http://www.sharepointblogs.com/mirjam/archive/2007/02/23/19983.aspx