IntroVisual Studio 2012 (an some earlier editions) include Schema comparison for SQL Server. This is NOT what I am writing about here today. Assuming you have used this tool to make tables, etc the same you may have a need like I do when moving data from dev to production databases or the reverse. There are several ways to go about this.
FREE - SQL Server Data Tools
Probably the best place to start is SQL Server Data Tools which is available from Microsoft. It includes among other things the ability to compare data in SQL Server tables.It is available for Visual Studio 2010 and newer. There was an option in some editions of Visual Studio 2010, but not in Visual Studio 2012. To get the functionality in Visual Studio 2012 you need the SQL Server Data Tools to be installed. Once you have it installed you will have functionality very similar to what was available in VS 2010 or the RedGate product. Here is a direct link for the download of the English ISO. One of the nice things about this option is that it is well integrated into Visual Studio 2012 and uses the same source and destination configurations as the SQL Schema Comparison that is built into VS 2012.You can also select what tables you want to compare, what columns in the tables, if you want source or destination records, etc. It will just to the update for you or you can have it generate the SQL Script that you can manually. It gives you a nice visual representation of the differences and let's you select the rows you want to change. It seems to be pretty fast. The generated SQL script even disables constraints as needed. It also seems to handle nulls properly. This is a very nice option for free!
It appears it can be called from the command line as well, but I have not tried it.
Here is the blog for the SSDT team.
FREE - tablediff.exe
IMHO, this may be the best choice for scripted options. Believe it or not tablediff.exe is a utility that comes with SQL Sever 2005 or greater. I believe this is the tool that SQL Server uses when replicating tables, though that is just what I read from someone else. It will tell you on a row by row and column by column basis what is different. It will even generate the SQL scripts needed to make the destination table look like the source table. As far as I know you cannot download it separately. However, it is installed when you install SQL Sever 2005 or newer and you choose SQL Server Replication feature. In SQL Server 2008 R2 it is included by default, but I'm not sure about the other versions. On my machine it was located at C:\Program Files\Microsoft SQL Server\100\COM\tablediff.exe. Once you find it you can type tablediff.exe -? for the options or refer here to the documentation. The parameters are pretty well documented and easy to follow to I won't go into all the options, but here is an example of how you would generate a change script (SQL) and see what the differences are.
C:\Program Files\Microsoft SQL Server\100\COM>tablediff.exe -sourceserver MySrcServer-sourcedatabase MyDevDB -sourceschema dbo -sourcetable Person -sourceuser User1 -sourcepassword User1Pwd -destinationserver MyDestServer -destinationdatabase MyProdDB -destinat
ionschema dbo -destinationtable Person -destinationuser User1 -destinationpassword User1Pwd -c -o c:\temp\diff.txt -f c:\temp\diffgen.sql
This will generate two files. diff.txt which will have the differences, and diffgen.sql which will be the SQL script you can execute to make the destination table the same as the source table.
Be careful, the SQL will also generate delete statements for your destination table. This may or may not be what you want so just be aware. I recommend backing up your destination table before doing this operation.
Also, it doesn't appear to generate correct scripts for null. It put null in single-quotes. This can be changed easily with a search and replace though.
Keep in mind this is per table. If you have lots of tables and you want them all to be updated it could be a done also, but it a bit tedious. However, the nice thing about this tool is since it is command line once you have it setup you can run it again and again with little to no effort.
If you want you can even download a GUI for the command line tool. NOTE: You still need the command line tool. I was not able to get it to work with SQL Server 2008 R2, but you can decompile it and fix it I'm sure. Does it work for anyone else? All it does is exactly what you do on the command line so I'm not sure I really see the benefit if you are going to want to run it more than once.
Inexpensive - SQLDBTools
If I have a little money to spend on the solution. SQLDBTools costs $60 at seems to do pretty much what the RedGate products do, but for a fraction of the cost. It does do schema and data comparisons which is nice to have it all in one tool. I does visually let you see the differences and it also generates the change script. I have not tried this product, but it looks like the best product for the money (if you are going to pay for a solution). Given that SQL Server Data Tools now exists I don't really see the advantage or reason to pay for this tool.
FREE - Linked Server
This is a less desirable option because it doesn't do anything automatically for you. After you have a linked server you can do queries to see the differences between tables. This does nothing for automatically generating the update scripts. This solution is good for analysis only.
Inexpensive - Beyond Compare
Beyond Compare does a nice job of visually showing the differences in two files. It doesn't know anything about SQL or databases, but as long as you use SSMS to export the data to CSV, Excel, or tabular data Beyond Compare will show you the differences in an Excel like manner. Again, this does nothing for generating the change script and could be slow for very large tables.This is best for comparing two adhoc queries in my opinion.
I found this site that has a pretty exhaustive list of tools for SQL Server that may be useful.