I use Eclipse's Database development plugins - like all Java based SQL editors, it works cross platform with any type 4 (ie pure Java) JDBC driver. It's ok for basic stuff (the main failing is it struggles to give transaction control -- auto-commit=true is always set it seems).
Microsoft have a decent JDBC type 4 driver: http://www.microsoft.com/downloads/details.aspx?FamilyId=6D483869-816A-44CB-9787-A866235EFC7C&displaylang=en this can be used with all Java clients / programs on Win/Mac/Lin/etc.
Those people struggling with Java/JDBC on a Mac are presumably trying to use native drivers instead of JDBC ones -- I haven't used (or practically heard of) the ODBC driver bridge in almost 10 years.
When this question was asked there were very few tools out there were worth much. I also ended up using Fusion and a Windows client. I have tried just about everything for MAC and Linux and never found anything worthwhile. That included dbvisualizer, squirrel (particularly bad, even though the windows haters in my office swear by it), the oracle SQL developer and a bunch of others. Nothing compared to DBArtizan on Windows as far as I was concerned and I was prepared to use it with Fusion or VirtualBox. I don't use the MS product because it is only limited to MS SQL.
Bottom line is nothing free is worthwhile, nor were most commercial non windows products
However, now (March 2010) I believe there are two serious contenders and worthwhile versions for the MAC and Linux which have a low cost associated with them. The first one is Aqua Data Studio which costs about $450 per user, which is a barely acceptable, but cheap compared to DBArtizan and others with similar functionality (but MS only). The other is RazorSQL which only costs $69 per user. Aqua data studio is good, but a resource hog and basically pretty sluggish and has non essential features such as the ER diagram tool, which is pretty bad at that. The Razor is lightning fast and is only a 16meg download and has everything an SQL developer needs including a TSQL editor.
So the big winner is RazorSQL and for $69, well worth it and feature ridden. Believe me, after several years of waiting to find a cheap non windows substitute for DBartizan, I have finally found one and I have been very picky.
My employer produces a simple, proof-of-concept HTML5-based SQL client which can be used against any ODBC data source on the web-browser host machine, through the HTML5 WebDB-to-ODBC Bridge we also produce. These components are free, for Mac, Windows, and more.
Applicable to many of the other answers here -- the Type 1 JDBC-to-ODBC Bridge that most are referring to is the one Sun built in to and bundled with the JVM. JVM/JRE/JDK documentation has always advised against using this built-in except in experimental scenarios, or when no other option exists, because this component was built as a proof-of-concept, and was never intended for production use.
My employer makes an enterprise-grade JDBC-to-ODBC Bridge, available as either a Single-Tier (installs entirely on the client application host) or a Multi-Tier (splits components over the client application host and the ODBC data source host, enabling JDBC client applications in any JVM to use ODBC data sources on Mac, Windows, Linux, etc.). This solution isn't free.
Squirrel SQL is a Java based SQL client, that I've had good experience with on Windows and Linux. Since it's Java, it should do the trick.
It's open source. You can run multiple sessions with multiple databases concurrently.
Try CoRD and modify what you want directly from the server.
It's open source.
I use the Navicat clients for MySQL and PostgreSQL and am happy with them. "good" is obviously subjective... how do you judge your DB clients?
This will be the second question in a row I've answered with this, so I think it's worth pointing out that I have no affiliation with this product, but I use it and love it and think it's the right answer to this question too: DbVisualizer.
I've been using Oracle SQL Developer since the Microsoft software for SQL Server is not currently available on Mac OS X. It works wonders. I would also recommend RazorSQL or SQLGrinder.
It may not be the best solution if you don't already have it, but FileMaker 11 with the Actual SQL Server ODBC driver (http://www.actualtech.com/product_sqlserver.php) worked nicely for a client of mine today. The ODBC driver is only $29, but FileMaker is $299, which is why you might only consider it if you already have it.
I've used (DB Solo) and I like it a lot. It's only $99 and comparable to many more expensive tools. It supports Oracle, SQL Server, Sybase, MySQL, PostgreSQL and others.
The Java-based Oracle SQL Developer has a plugin module that supports SQL Server. I use it regularly on my Mac. It's free, too.
Here's how to install the SQL Server plugin:
I have had good success over the last two years or so using Navicat for MySQL. The UI could use a little updating, but all of the tools and options they provide make the cost justifiable for me.
When this question was asked, Microsoft's Remote Desktop for OS X had been unsupported for years. It wasn't a Universal Binary, and I found it to be somewhat buggy (I recall that the application will just quit after a failed connection instead of allowing you to alter the connection info and try again).
At the time I recommended the Open Source CoRD, a good RDP client for Mac.
Since then Microsoft Remote Desktop Client for Mac 2 was released.
I thought Sequel Pro for MySQL looked pretty interesting. It's hard to find one tool that works with all those databases (especially SQL Server 2005 . . . most people use SQL Server Management Studio and that's Windows only of course).
I vote for RazorSQL also. It's very powerful in many respects and practically supports most databases out there. I mostly use it for SQL Server, MySQL and PostgreSQL.
I've used Eclipse with the Quantum-DB plugins for that purpose since I was already using Eclipse anyway.
I like SQLGrinder.
It's built using Cocoa, so it looks a lot better and feels more like an Mac OS X application than all the Java-based application mentioned here.
It uses JDBC drivers to connect to Microsoft SQL Server 2005, FrontBase, MySQL, OpenBase, Oracle, PostgreSQL, and Sybase.
Free trial or $59.
Not sure about open-source, but I've heard good things about http://www.advenio.com/sqlgrinder/ (not tried it, I prefer to write Python scripts to try things out rather than use GUIs;-).
Ed: phpMyAdmin is for MySQL, but the asker needs something for Microsoft SQL Server.
Most solutions that I found involve using an ODBC Driver and then whatever client application you use. For example, Gorilla SQL claims to be able to do that, even though the project seems abandoned.
This doesn't specifically answer your question, because I'm not sure in any clients exist in Mac OS X, but I generally just Remote Desktop into the server and work through that. Another option is VMware Fusion (which is much better than Parallels in my opinion) + Windows XP + SQL Server Management Studio.