Mac: How to mount a Windows shared folder

Set up sharing on your PC. To get this going, make sure your PC is set up for file sharing. In Windows, choose the folder you want to access from your Mac, and right-click to get to its Properties dialog box. From the sharing tab, turn on sharing, and choose a name for the shared folder. (This is the name you'll see on your Mac.)

To be safe, stick with something less than 12. Here, I'm sharing the "My Documents" folder as "PC - My Docs." Also, check the "Allow network users to change my files" if you want to save files to this share from your Mac.
Determine your PC's IP address. You'll need to give your Mac an address to look for this shared folder, so get your PC's local IP address from the command line by running ipconfig. In my setup (shown below), that address is 192.168.1.102. Write that number down.

UPDATE: Several readers have pointed out that you can use your PC's name versus its IP address to connect to it. Using the name is a better method than IP address because the IP can change if it's assigned dynamically, but the name won't unless you change it.
Connect to the server on your Mac. Now, on your Mac, from Finder's Go menu, choose "Connect to Server." Enter the IP address you just obtained from your PC, preceded by smb://, as shown:

UPDATE: To use your PC's name instead of IP address, replace the number with the name. So if your PC's name is "MyWindowsBox", you'd use smb://mywindowsbox (not case-sensitive).
If it finds your server, you'll get prompted to enter your network credentials - by default, your Windows username and password:

Once you're in, you'll get to choose which shared folder you want to mount on your Mac. In this case, we're going to choose the "PC - My Docs" share:

And you're done! Now the "PC - My Docs" folder will be mounted on your Mac, and accessible from your desktop and in Finder. If you've allowed reading and writing to the shared folder in Windows, you can drag and drop and save files to it as if it were any disk.
This technique comes in very handy for backing up your Mac's data to a PC, too - just set your Mac's backup software destination to the Windows drive.
UPDATE: You can automatically mount your Windows drive every time you login to your Mac by adding it to your Login items. In System Preferences, Accounts, choose the "Login Items" tab and add the Volume to the list of apps that automatically start up.