This is a simple hack which allows one to charge a custom tax rate based on a member's City and State.
It makes no changes to the database or the normal tax structure in anyway. It simply checks for a customer's entry_city in their address book and then compares it to their Zone Id, or State/Province, and returns a custom tax rate and description that you input directly into a single file. This is a simple hardcode workaround, and is not an accessible feature through the Admin menu. It's simple to install though, and will solve your local/city tax problem. It could also be modified to look at the entry_poscode and entry_suburb fields of the address_book table, if one wanted to use it for zip codes or suburbs.
Enjoy!
Legend: 
Download

Report
In North Dakota and South Dakota there are Zip Codes that include citys that charge a sales tax. Some Addresses within a Zip Code require city sales tax and other do not. I have made changes to enable charging sales tax by Zip plus four code. This mod also allows for the tax rates to be stored and maintained in the tax rate table in Admin.
Using the information from the previous mods posted here I was able to create the logic to allow my Store to compare the customers shipto Zip code to the tax rate table and charge them the correct sales tax accordingly.
I too found myself running osCommerce in a State that requires me to not only collect Sales Tax but also a County
Tax. In PA Philadelphia and Allegheny Counties each charge residents a 1% Tax.
Using the information from the pervious mods posted here I was able (with the help of a friend) to create the
logic to allow my Store to read ZIP Codes from a file, compare then to the buyers information and charge them
the correct sales tax accordingly. I have also included all the ZIP Codes from Philadelphia and Allegheny Counties
so anyone using this mod in PA can just install the files and not worry about anything.
Enjoy!
This is the same code as previous, but with the multiple logic statements that allow you to control different tax rates in your specific city, county or state. Fully compatible with 2.2.
This is the same hack as I posted below back in 2005. However, I have added some additional instructions on how to add additional zip codes and zones.
For anyone new to programming PHP the text at the bottom of the file should help you utilize 'else if'statements to cover whatever range of zipcodes and zones you need.
Also, thanks to Jeff Allen for motivating me to update this. :)
Nathan
Available in this package are tax rates for all the cities in California. It's a complete package - you don't need to download any of the other packages of this contribution too. It's current as to August 2005, meaning if you download it too much later than that, you'll want to check to see how many cities have changed their tax rates.
This 'hack' calculates the state and city tax rate for a location based on the recipient's zip code.
Like the 'City Tax Rate Hack' on which it was based, this hack utilizes a hard coded workaround.
In particular, is solves the problem I was facing of customers with multiple in-state addresses not being taxed correctly.
This is a simple hack which allows one to charge a custom tax rate based on a member's City and State.
It makes no changes to the database or the normal tax structure in anyway. It simply checks for a customer's entry_city in their address book and then compares it to their Zone Id, or State/Province, and returns a custom tax rate and description that you input directly into a single file. This is a simple hardcode workaround, and is not an accessible feature through the Admin menu. It's simple to install though, and will solve your local/city tax problem. It could also be modified to look at the entry_poscode and entry_suburb fields of the address_book table, if one wanted to use it for zip codes or suburbs.
Enjoy!