PhpMyAdmin for Sql Server

Yes, there's the new Database Manager application:

http://www.iis.net/extensions/databasemanager

At Mix 2009, Microsoft announced this and demoed it. I haven't used it myself, but the demos were gorgeous, and since it's maintained by Microsoft, it's probably the safest bet.

Weird that no one mentionned myLittleAdmin. This is a 13-year-old project with a robust and stable kernel. Its interface is very similar to SSMS one. And of course it is a full web-based app.

SQL Web Data Administrator may be sufficient for what you're looking to do, but it hasn't been updated in about a year and a half. If you need more info, there's a review by SQL Server MVP Jacob Sebastian on SQL Server Central.