This plugin, once activated, generates a sitemap.xml on-the-fly for every site in a multisite network.
As the title says - simple, but good :)
Not only does this plugin "just work", but it "just works" on WordPress Network installs too which makes this plugin way better than all the other XML Sitemap plugins for WordPress.
Simple, no option, clean.
I tried one of the usual sitemap plugins but found that it didn't work with multisite. So I uninstalled and installed this one instead. All 3 of my sites now have unique sitemaps - as they should. Works great!