Great question.
First of all lets take a look at the value of what we provide in SQL Server, I'll focus on SQL Server 2005(Yukon) but much of what I will say applies to SQL Server 2000 as well.
We have a truly free offering, its called MSDE several months ago we changed the licensing so that anyone can download it and use it in production. MSDE is the desktop version of SQL Server, it includes almost all of the functionality of the SQL Server relational offering with the restriction on workload and database size.
In Yukon we are doing some of the deepest thinking about the MSDE/Small/Medium customer that we have done since I joined the team, it has been challenging and fun. You should here the results of our work announced with SQl Server 2005 Beta 2 in the summer. BUt I think the existing customer base will be very happy and we will bring a lot of new folks into the fold.
Next lets take a look at the value of SQL Server Std; In the box you get SQL server Relational Engine with support for all of the features I was talking about plus TONs of others, in addition you get our comittment to quality, performance and security. These 3 principles apply to all of the box that also includes; Analysis Services(currently the number 1 OLAP server in the market), Reporting Services( which in its SQL2000 implementation has proved to be a HUGE success), Data Transformation Services(used by almost 80% of SQL Server customers), Full Text Search, XML/XQuery/SOAP etc etc etc.
Essentially SQL Server in Yukon is an app platform in a box, pretty much any type of Data Centric App that I would want to build can be built on top of Yukon, and by using Visual Studio 2005 the app can be built in a very productive and rich way.
With our Enterprise Edition we can provide performance for multi-terrabyte systems at %x9's level uptime, that supports the biggest apps and companies in the world.
So SQL Server is available from the desktop to the highest end of the enterprise, with the same code base, same programming model.
I don't want to focus on other products too much but I suggest looking into the real costs, especially over a 3 yr timeframe, for many of these other products you actually need to pay to use them commercially, especially if you want 24x7/Escalation support. And if you look at the way these products are managed over time they are usually more expensive than SQL server if you take ALL the costs into the equation rather than just the software aquisition costs.
BTW if you are not a SQL Server customer today I'd love to hear what you are using, what you are doing and why you choose the platform you did.