Just to add to this, ES File Explorer also does this, plus bluetooth support and free from the market.
However another method i like to use, is rather than grab files from the PC as you stated; push the files from the PC to the phone. That means i dont need to touch the phone to get files - I can do it from the comfort of my PC.
To do this, install FTPDroid on your phone. Under settings, click WiFi AutoStart so that it will start the FTP service whenever you are connected to WiFi (in my case, only at home or work).
By default it enables the anonymous user, which might make it easier to connect from the PC, but would be a security risk if someone were to connect to you while you are at a public wifi. This can be managed by unticking the anonymous access in the settings, then adding your own user (from the main screen, press menu, manage users)
You can further automate the process by installing WiFi on AC Power or Auto WiFi, so that your WiFi, and hence FTP site start automatically when you get home
You can configure your router to automatically give the same IP address to your phone each time, but in my case it hardly ever changes. Once the FTP service starts it will show a message on the screen saying "FTP Server active on ftp://<ip address>:2121 (port 2121 is default)
From the PC you have a few options:
1. Use a dedicated FTP client to upload/download files
2. Add this address to the address bar of Explorer in Windows
3. Create a shortcut on your desktop; when given the prompt of what program to start, type:
explorer.exe ftp://<your phones IP address>:2121
4. If you dont know how to fix the IP address given to your phone from your router, you can create a small batch file like this:
set /P IPAddress="Enter the last segment of the IP Address: "
^^^ Paste this into Notepad, then save the file on your desktop as My Phone.bat (ensure filetype is set to *.* all types). Note that 192.168.1. should be replaced with the first 3 segments of your network's IP address
When you double click this you will be prompted for the last segment of the IP address - something you should be able to type pretty easily.